Roger Cox: :Robin Harvie, author of Why We Run, on his experience running the Spartathlon

"At about ten o'clock at night I started hearing voices," he says. "I'd been running for 15 hours by that point. Then, the more fatigued I got - and I think this is very common for distance runners - the more I started seeing shapes appearing at the side of the road.

"There was a thunderstorm up ahead so there was a very peculiar light being thrown down into the valley, and I started seeing dogs and cats and people..."

Jumping out at you? "No, just sitting there at the side of the road. Of course, as soon as I got up close to them they disappeared."

Harvie ran his first marathon in London in 2000, and he describes himself very modestly as "a quite ordinary runner". After being bitten by the long distance bug he competed in more and more marathons but never seemed to be able to beat his usual time of three hours 30 minutes. After completing the Paris Marathon in 2006 he thought "hang on - I don't feel that bad - if I can't run any faster, how much further can I run? Five miles? Ten? Could I do the whole thing again?"

This curiosity about where his limits might lie led him to compete in the Spartathlon, the daddy of ultra-distance foot races, set up in 1984 in honour of the Greek messenger Philippides, who was sent from Athens to Sparta to get help when the Persians landed at Marathon in 490BC. Harvie's Greek experiences form the basis of his book, Why We Run, and he will be reading extracts from it next weekend at The Scotsman's Outdoor Pursuits show at the Royal Highland Centre.

At the Spartathlon start line in Athens, Harvie got chatting to a fellow competitor, a veteran of 15 Spartathlon finishes, who told him success in these events was "90 per cent mental". Which is why, when he started seeing things 85 miles in, he knew he was in trouble. "At the time it all came to a head I found myself by a tiny chapel in the middle of nowhere," he says.

"I sat down at a checkpoint and ended up puking up all over myself. Then I took my shoes off and saw my toenails had started popping out and there was blood all over my feet."

Unsurprisingly, perhaps, Harvie decided to throw in the towel at this point, and he was eventually shuttled to the finish line on a bus full of other shattered runners who, he says, "looked like they'd just been pulled out of the trenches." The following day he could barely walk, but his mind was undergoing yet more unusual changes, and for several weeks after his ordeal he says he found himself in "a state of grace".

"I could feel that I was a much calmer person," he says. "Empathetic is the word. I was much less likely to rush headlong into things and much more likely to be patient and calmer and quieter than I had been before. I just felt like a better person."

Does he think it would be possible to achieve the same effect through short bursts of moderate exercise?

"No, I don't think you could really run on a treadmill for half an hour and think that was enough," he says. "I think you've got to push yourself to the point where you're almost split in two. When I was in Greece, I saw something of myself then that I'd never seen before. It was a very, very humbling experience to be broken like that and then to be able to walk away. I think you've got to do that - push it right to the very extreme. And the point is you can go back and do it again - it's there for everyone to experience."

Why We Run is published by John Murray on 14 April; The Scotsman's Outdoor Pursuits show, in association with Rat Race Adventure, is at the Royal Highland Centre, Edinburgh, next weekend. Highlights include stunt bike displays, bushcraft workshops and snowboard simulators. Tickets cost 10 for adults, 1 for children, aged 5-16, visit www.scottishoutdoorpursuits.com

• This article was first published in The Scotsman on March 26, 2011