Written by Robin Harvie - Extract from Why We Run published in The Telegraph

If the marathon isn't far enough, there is always the Spartathlon: 152 miles in less than 36 hours. In his book Why We Run, Robin Harvie describes taking up the challenge

Thirty-six hours? Non-stop? Impossible! I returned to the maps of the Peloponnese again and again, as I knew all the competitors were doing, to trace with my finger the contours we were going to cover. The maps made me feel like a professional, but with time they simply revealed more clearly the insurmountability of the challenge.

Every detail had found its place in the notebook I had kept throughout the year: speeds, distances covered, weight lost and gained. There were also training plans, dietary requirements, logged times, sketches of the races to compete in as preparation. I had worked out the skeleton of an itinerary that covered everything from how to treat my feet before my shoes went on to what time to arrive at the start line and what food to eat as I ran. Over the course of the race there were 72 numbered water stations, roughly 4km apart, where I would leave bags with dates, nuts, goji berries, fruit pastilles and carbohydrate powder. One water bottle would be strapped to my back, a heart rate monitor to my chest and a speed monitor to my belt. There would also be a mobile phone, sunglasses and a sunhat. There would be food at most of the checkpoints, and a head torch waiting for me at the 109km checkpoint for the night section, with a change of socks and a fluorescent jacket.

Every conceivable eventuality was committed to paper, leaving nothing to chance. Yet no amount of preparation could tell me what 36 hours on my feet would feel like. Since its conception in 1984, no more than 700 people have ever completed the Spartathlon. To emphasise just how tough this race was, I remembered that even those who did not finish were serious ultra-distance runners, since the minimum criterion for qualification was completing a 100km race in under 10½ hours. (Which I had done, in Amsterdam, just – in 10 hours 19 minutes.)

I arrived in Athens 10 days before the race. My wife, Laurence, and I took a boat to one of the islands and for a week I jogged on the beach, acclimatising to the heat. Every day I would wake to watch the Mediterranean sunrise, thinking forward to the moment when we would stand at the foot of the Acropolis. As the temperature rose during the day, I speculated that if I could get past 3pm and the worst of the heat, I might have a chance.

We did calculations on the back of restaurant napkins, working out how fast I would need to run to get to Corinth, the first major checkpoint, in the required 9½ hours. Each morning in the sea I cleaned, filed and sanded down my feet to make sure that there was no loose skin or hard surfaces that would be exposed to abrasions. 'Don't think that I don't understand,' Laurence said. 'You want to do something extraordinary. I see that. But just once, then enough. We have a life to get on with, and I don't want to do that alone.'

On the afternoon before the race, I arrived at the Hotel London in Athens for final registration. This is where the logic of wanting to run a marathon really ends: with 400 of the best ultra-distance runners in the world crammed into a one-star hotel on a busy motorway. Everyone knew that fewer than 30 per cent of us would finish, but for the time being that was all ahead of us. We were still invincible.

The weathered faces of the runners, mostly in their forties and fifties, told of years spent on the road – rather than the 12 months I had spent running 120 miles a week: 6,000 miles in total. Bare feet revealed sock-level tan lines, and deep grooves around the eyes spoke of hours gazing transfixed at the horizon. The age of these runners indicated that they had tired of marathons. It also suggested the maturity required to spend this amount of time on your feet. All but the elite need two or three attempts to finish the Spartathlon, and after each race those who do not finish come back the following year, with a determination not to be broken again.

They were not professional athletes, since the Spartathlon prides itself in not offering any cash prize to the winner. The competitors are the people who guard your prisons, put out your fires, decorate your homes and deliver your post. For many it is not a pastime but a way of life.

Looking lost in the hotel lobby, I was befriended immediately by Gilles, a French veteran of 10 Spartathlon finishes. He put his arm around my shoulder when I told him it was my first time and smiled. 'La première fois, c'est très dur!' Gilles had run 14 200km races since April and he made it clear that, with less than 24 hours until the start, a competitor's success or failure was predetermined by a training plan that dated back as far perhaps as Christmas Day. All these runners knew it and there was nothing that any of us could do now except stand around and wait.

Gilles said that he planned every holiday around running races, 'the cheaper the better', and spent his entire year training for the Spartathlon, the purest running race in the world and the hardest. The rest, especially the Marathon des Sables, was 'pour les frimeurs', show-offs. The latter was not even that tough, since it took place over six days with time to sleep between stages. The Spartathlon, by contrast, is a deliberately fast race with strict time limits for leaving each of the 72 checkpoints. For some the greatest hurdle is the heat, for others it is running through the night, but for every competitor there is no possibility of stopping, no time to compose yourself if it starts to go wrong.

The race starts at the foot of the Acropolis at 7am. Gilles wandered between the runners, wishing us all luck. He saw me as I walked to the back of the group to perform some futile last-minute stretches. He had thought my idea of treating it like running two 4½-hour marathons to Corinth, a good one. 'Patience! Patience! Patience!' he said, raising his finger to emphasise the importance of his advice.

The bell rang and shouts of joy went up: 400 runners wested out of Athens down the steep slope of the Acropolis, as a weak sunlight stole over the landscape, like a thief through the window someone forgot to close. I was the last to cross the start line and gave one final wave to Laurence. I would not see her now until Sparta.

There is no neat equation that explains why people choose to put themselves through the trial of running these distances. For some, like Gilles and Thierry, with whom I ran for the first seven hours, it was a way of releasing the coiled-up tension in a personality that was so tightly wound that to imagine them not running was impossible. For others, like Mark Cockbain, who arrived at the hotel with a bag slung over his shoulder as though for a beach holiday, running these distances exfoliated the junk of everyday life.

The road to Sparta had started with a simple question – why do we run? I had spent a year training for the race, but had also been trying to understand better what compelled me to do it. At its most public, we run for the competition and the adventure. However, the more I ran, the more I realised that for me it was a far more personal epiphany, anchored in pain and revelation, but even these motivations were difficult to fully grasp. Perhaps there is no answer. This is just what we do, and running these distances offers a richness of experience that contributes to our understanding of not only ourselves but also of the landscape and our own personal history.

Behind me I saw it for the first time – daybreak. A cool, limpid sunrise, little more than a lightening of the eastern haze. As the first hours passed, we ran against the flow of commuter traffic and through the bulging industrial estates on the outskirts of Athens, before turning off the motorway to weave along the old coastal road to Corinth.

Clothed, these runners looked startlingly humble. We can see the true strength of someone only when they are stripped down, at which point these men suddenly became supermen. The sinews and muscles on the backs of their legs ricocheted with every step. Even with their arms hanging loosely down by their sides, each particle of tissue and muscle, sharply defined, flexed like rippling sheets with every movement. I had never been this close to the sensuality of the athletic form before. In all the time I spent with the runners of the Spartathlon, there was never any sign of the vanity acquired by the first-time marathon runner. All aesthetic perfection had long ago been put to one side, as though their physical manifestation as runners was incidental to their cognitive being.

'Take care of what happens up here,' Thierry said, tapping his temple with his index finger, as we slowed to a walk over the first climb. He informed me that the body is little more than a vehicle for carrying the will. I looked at him doubtfully. 'All you need to do with the legs is more of this – boom, boom, boom,' he said, indicating the motion of running. 'When the mind starts to go, then you know you are finished.'

With the hottest part of the day, which peaked at 42C, came the water as we joined the coastal road. To our right, white cliffs thrust away to a blue sky and to our left fell into a sea that breathed quietly, like a sleeping dog. I was not alone in yearning to strip off completely and plunge in headlong, and I passed a Japanese runner clutching an ice pack to his head while looking over the edge longingly.

The mood at Corinth (80km), where Thierry sat down in the shade and tore open a sachet of rehydration powder, dismayed by his body's failure to deal with the heat, carried me through the rest of the day. He shook his head when I asked whether he was carrying on.

Fifty of those who had set off ahead of me in Athens had handed in their numbers by the time I left Corinth. I was surprised to find that although my swollen feet ached and my legs had warmed to that feeling of pleasant discomfort, there was nothing in the muscles yet that was telling me to stop. With Corinth behind us and the first psychological barrier having winnowed out the day's weakest runners, a greater distance began to stretch between those who were left. Occasionally a runner would pass me, nod and carry on. By the time I reached the next checkpoint, they had already either moved on or given up. By 8pm the remaining light had faded completely. Most of us had yet to pick up our night gear so we had to grope along as best we could.

At the 109km checkpoint I sat down to accept the offer of soup and tea, then rewarded myself with a change of socks and clean T-shirt before putting on my fluorescent jacket and strapping on a head torch. This was 9km further than I had ever previously run and amounted to 13 hours on the move without stopping. For the last hour I had run in the shadows, unnoticed, of two pale Finns, making me buoyant again at the thought that I was not alone, even though I had not exchanged a word with anyone since leaving Thierry four hours earlier.

I had left Corinth in better physical shape than I had been in Athens, with all the nerves and junk of preparation finally flushed out. As I passed through the significant checkpoints at 90, 100, 109, 120 and 125km, although I was walking the slopes more than I had during the day, my legs still felt indestructibly powerful, and my feet had only mild abrasions.

Time passed more slowly now. As we moved further into the darkness, I started to become troubled by a genuine fear that was crystallised by the knowledge that it would not get light until shortly after 7am. I was set for the longest night of my life. Gilles had talked about this moment and had gripped my forearm, telling me that I must hold my nerve. 'Now is the time to be brave,' he had said, echoing the traditional phrase of the French executioner as he entered the condemned man's cell.

After the 120km checkpoint, the bouts of melancholy became more frequent. In real terms nothing had changed. The Finns were still 50m ahead, while I was as rigidly disciplined as I had been all day in keeping my pace down, walking the slopes and listening to my body's demands for more food and water. But every runner will tell you of those moments of despondency, the inescapable desire to be somewhere else that, over time, starts to grip more stubbornly. There was no one to stop me from quitting, and I was in part carrying on simply to see when the last step would come. What I had never seen before was the physical and emotional limits of my own capacities. For the first time in my life I was living at the absolute extremity of my own being, experiencing a frisson of danger in this discomfort zone.

Every runner speaks of the surprise at how quickly bodily sensation can alter and the feeling of aching comfort transform itself into a pall of disconsolation. We had reached the foot of the mountains now and the sky was utterly dark, the moon weak, a dirty brown behind the clouds. Ahead of me the Finns turned off the main road back on to the mountain paths and we heard the promised thunderstorm cracking ahead of us. They too were suffering, I could see. Their legs reached down and forward with a slow pedalling movement, like a man descending through the trapdoor of a loft and feeling for a ladder with his feet. They were running with a clumsy arthritic tread; it was the gait of the dying, yet pathetically funny to watch. But they were still heroically pushing forward, and I had to stay with them.

By 10pm, after 16 hours on the road, I was starting to meander as I began to hallucinate. Andy McMenemy, who had run the Spartathlon the year before, talked of seeing seals flapping past him that seemed so real he would step aside to let them pass. What I needed, I realise now, was someone to talk to, some noise to fill my head. I could feel part of my mind falling away from me like the slow descent of the side of a house under a wrecker's ball.

Checkpoint 36 marked exactly halfway between Athens and Sparta. I arrived at 11pm, nearly 40 minutes ahead of the closing time, and sat down in the crowd of runners and supporters who were cooking meals on gas stoves or peering at graphs on laptops. A few runners, whom I had not seen since the morning, huddled under blankets. I was handed a bowl of pasta and ate it with a knife picked up from the ground. The Finns were sitting quietly out of the way, massaging their calves and saying little. Five minutes later I was up and out of the checkpoint, thinking that they had already left. The road itself was indistinguishable but the lonely traces of head torches indicated a path that wound up towards a peak that was just visible in the moonlight. I was relieved. There was no way I was going to run that, so I zipped up my top and started to speed-march in search of the Finns.

Half an hour after leaving behind the empty pasta bowl, I started to question how much further I could actually go. It was 11.30pm and the mountains were now starting to become an issue. As the minutes and then the hours accumulated, the more frequently I looked at my watch, the more I could feel each second form its own character before lurching on to the next. It was not just that I was slowing down; time itself seemed to have expanded, opening itself up to scrutiny at the atomic level.

The Finns were nowhere to be seen, and I realised too late that I had set off ahead of them. Running past the bus that would carry those with DNF (did not finish) added to their name only made it worse. I was sinking deeper into the warm milk. I could just stop. Here. And step on to the bus. An invisible force was crushing me. I could feel its weight, its hypnotic powers. Wherever this force had come from, it was now inside me: it could dissolve my will and cause my heart to stop beating. Only people who have never felt such a force themselves can be surprised at how quickly others submit to it. Those who have felt it, on the other hand, express astonishment that a man can rebel against it even for a moment.

Faced with real suffering for the first time in my life, I speculated momentarily on my chances of going to heaven. But even as I thought it, I knew this was another trick of my disintegrating mind. After all, I could just stop; it was not as though I was stuck on a frozen plateau miles from safety or stranded on an exposed peak, although my isolation and peril felt real enough.

The road seemed never to stop going up. I couldn't believe this to be true since it bore no relation to the graph of the changing altitude of the course that I had pinned to the wall at home. What this graph showed was that mathematics had only a superficial relationship to the outside world, and when addressed like this it had no correlation to humankind. The long, lone ridge opened across the skyline like the low hull of a submarine. Through the solitude I was no longer forcing myself from within, but was being lured through that horizon and on to others. They disappeared from view and from memory. There was no moon now, only black asphalt that extended to the stars, with the next checkpoint another four inconsolable miles away. I had the feeling that I was capsizing, my inner ear disorientated from motion sickness, and I was sure that I was about to keel over at any moment.

After 17 hours, it all corroded in an instant. I could not breathe, could not move, was too weak to talk or to think, or even to be dismayed as a cherished fantasy dissolved. Glued to the road, in the space of 10km I had turned from indestructible to an invalid. I stared ahead into the dark, trying to imagine the finish line, something I knew I should never do. My head felt impossibly heavy and rolled on to my chest.

The place where I collapsed was like the scenery from the Middle Ages. I would like to think that, with poetic intuition, I understood that in the darkness as midnight chimed, it was the end of a dream, that there was a neat closure in not being able to run on into the next day. But all I could think about was how relieved I was that I didn't have to run any further.

The table at Checkpoint 38 – 134km – was set up by the side of the path at the entrance to a small chapel. It was midnight. I sat down on a plastic chair facing a bush and tried to think what I was going to do. As I did so, I felt part of my brain rushing forward and my stomach crashing upwards. No one was surprised to see me throwing up. Out of inexperience, I had been taking on large amounts of water, even though the temperature had dropped dramatically, and all the careful preparations I had made were negated in an instant.

I had 40 minutes until the checkpoint closed. Almost immediately after I had been sick, my body started to shut down. I knew from Mark Cockbain that this would happen. I still had plenty of time to collect myself, but my eyes were closing and I began to shiver. 'You won't feel any worse,' he had promised. 'Just remind yourself of that.'

I felt drunk, elated but uncoordinated, entranced and incoherent. I tried to stand up but my legs gave way beneath me. 'Enough.' I gestured pathetically to the race organiser. 'I'm finished.' But it came out as a breath of air too weak to be heard the first time. 'Excuse me. I've stopped.' Everything changed suddenly. They asked me twice if I really was going to quit and I was told I needed to hand in my numbers. And please sign here. 'We'll take you to the bus.' 'Thank you,' I mumbled.

The Finns arrived as the Land Rover appeared, kicking up dust, and they politely asked for a cup of tea. Hunched over the long stilts of their legs, they stood motionless, feigning death, their faces pale in the light of the oil lamp, macabre and sorrowing, like those of drowned men. I knew that this was how I looked too. The Finns, however, were not finished yet, since they had not made the mistake of going off too quickly at the last checkpoint, and soon they were on their way. The bus was full of the dead and the dying, slumped across every seat revealing the sub-soil of human nature. Legs were curled up, covered in dust, salt, vomit and blood. The bandages and the popping veins would have taken hold of anyone's imagination, but unless you have been there you cannot know how profoundly humbling it is to feel that you have been thrown up by the sea, battered, limp and lifeless. There was no violence in the exhaustion that is felt over shorter distances, even marathons. Beneath the bare and glaring lights, all the runners who had quit before me lay asleep as though dead drunk. Some of them I had not seen since the start line. Others had passed me by silently hours before. By the time I finished, 180 others had quit.

During the day I had heard that people had quit for dramatic reasons. One runner was insisting that he would carry on, only to be forcefully returned to his seat, told that he was in shock and would be put in the back of an ambulance. There was one cracked skull and multiple cases of dehydration. Very few dropped out between here and Sparta, suggesting that, theoretically, I had got over the worst of it and that, if I had carried on, statistically I would have stood a high chance of being one of the 130 who finished on time. But I had learnt the hard way to be honest with myself. I could not have run any further. I gathered my belongings and crumpled against the window as the exhaustion entered me through my closing eyes.

We arrived at the hotel in Sparta at 3am. Laurence found me in reception. An Italian was trying to get me to drink some flat Coke, as we sat around waiting to be told what to do. Ushering me quietly into our bedroom, I followed her biblical instructions: take, eat, as she tried to get me to open my mouth wide enough to put in some bread and jam, but could get only a corner in. Then she helped me through the basic mechanics of undressing and getting into bed as Friday, September 24 2009 was finally snuffed out.

How can I describe the deep, vibrant pleasure I felt in those weeks afterwards? Perhaps it was a little like the moment just before a child bursts into tears. He knows that he is going to cry and he does nothing about it. He has no shame, he wants to drown in it, to be swallowed up by his emotions. As the fatigue receded, I was less able to hold on to the precise memory of exactly what I had been through. The warm, numbing calm stayed with me longer, and yet, in time, that went too. I found that the saddest of all, since I had hoped, too optimistically, that once I had achieved that state of self-obliteration, it would remain with me permanently, lifting me above the grubby banalities of everyday life.

For three months I slept for 14 hours a day. My body gradually healed, the swelling in my feet subsided, the blisters burst and blackened, and by Christmas my legs had eventually lost their dumb ache. I had known beforehand – because I could see it in the eyes of those who came back year after year and still had not finished the race – that to quit before getting to Sparta meant to be left suspended, eternally perhaps, between two points, able to go neither forwards nor back. However, in those first months I was convinced that, even though I had failed, I had satisfied the contradictory and conflicting motives that had compelled me to attempt to run from Athens to Sparta in the first place. I thought I could get on with the rest of my life. With that conviction came a real sense that I was finally at peace with myself, that I could calmly return to my sedentary existence a wiser, better person – for ever.

But soon enough, the silt of everyday life returned – the soft and sticky slowing-down of both motion and thought. These were the first signs that I was starting to lose my grip on the knowledge, the state of grace, that I had found on the road to Sparta. I became impatient, restless and boorish. I could feel the film of grime returning to my skin, which was not surprising since I was not running, sweating or showering nearly as much as I had done the previous year. This was how I had perceived the world before the race, and I did not like what I saw. I put on weight, I started to drink again. As time drew me further away from the mountaintop in Greece, I began to feel as though I had to wade through the day just to keep up.

While, in part, this was my old self returning, my unhappiness also came from knowing that I had left something of myself up at Checkpoint 38. It was not long before the lust to return to Greece started to gnaw away at me. It came most strongly during the night, perhaps most obviously because my body was still recovering, and violent twitches in my sleep would wake me. I had promised Laurence that I would never go back, and it troubled me deeply to wonder how long I could maintain that fiction. Comfort came in the knowledge that others were thinking the same too. Gilles had told me that he had started planning his second attempt within days of returning to France. Once we had scattered back to our corners of the world, I liked to imagine the other runners waking in the dark, as winter encroached, thinking about how they would finish the race next time. And the tragedy was that we all knew that, until we had dealt with that unfinished business, we could never be fully satisfied. It was as if we had been shown a glimpse of a route to eternal contemplation, but had been told, 'Not yet!'

What I learnt in the months that followed was that when I returned to the Spartathlon, as I knew I would, I would have to do it not to tell a story about a journey into adulthood, or to excavate the reasons why we run. I would have to take it on, silently, simply for the sake of running itself.

Through the process of identifying the many elements that make up the inclinations of the runner, I wanted to show that there is a story that unites us all – the elite, the ultradistance runner and those who stick stubbornly to their five miles and no more on a Sunday morning. I suppose that, in this process, I had hoped that a coherent reconciliation would emerge of why we do this, day after day, week after week. By starting with the familiar and digging towards a primary bedrock of a universal explanation, one that we all know is in our hearts but may not have articulated well enough for others to understand, I hoped to give some kind of answer to the question: why do we do this? As you may have guessed, there is no neat answer. There is no point at which we stop. We just keep returning, giving ourselves over to these huge distances, because this is what we do.