Their findings, which Runner’s World magazine published a summary of, concluded that his ability to run for great distances without suffering from muscle damage was the result of decades of practise rather than a genetic advantage or freakish physiology. “Karnazes’s ultra marathon habit has built up his bone-density, joints, running muscles and blood transport system to the point where he can motor along forever at a seven-to-ten minute per mile pace.”

But can a man who can achieve such things really be like us? And if he wasn’t genetically predisposed to it, how did it start, this “ultra marathon habit?”

Constantine “Dean” Nicholas Karnazes was born in California to Greek immigrant parents in 1962. Both his father, who worked as a field naturalist, and his mother were outdoorsy types, and were supportive of his adventurous streak from a young age. “I could just go and do whatever I wanted all day,” he says. “There were no rules really, they just kind of let me do my own thing.”

“What’s truly impressive about Karnazes, is actually that his body isn’t particularly remarkable”

At one point, he famously cycled more than 40 miles from home to his grandmother’s house, despite not really knowing where she lived. “This was before iPhones or whatever, I didn’t know where I was going. But then I got somewhere close and I recognised this one restaurant. I showed up at my grandmother’s house, and she was shocked. She called my mum and she couldn’t believe it,” he laughs. Dean was 10 at the time.

At junior high school he turned his efforts to running, first completing the full marathon distance at the age of 14. It was a fundraising event where each lap earned a dollar for charity. Most kids ran a few laps and gave up, but Dean just kept going.

But although he evidently loved it from a young age, his athletic progression was abruptly cut off in his teenage years. He discovered alcohol while still in high school and took to it all too enthusiastically. In Ultra Marathon Man, Karnazes says he basically stopped running for 15 years, only starting again on his 30th birthday.

He had a similarly strange experience during the feat of endurance which forms the centrepiece of his recent book – a 153 mile run between Athens and Sparta in Greece known as the Spartathlon. “The toughest thing was sleep deprivation. There was a point where I was running and there appeared to be a praying mantis, a little stick figure, in between my feet down on the ground. I couldn’t put it into perspective. I was like: ‘What is that?’ And then I realised: ‘Holy Shit, that’s me!’ I was out of it.”

The Spartathlon is based on a run supposedly undertaken by an Athenian hemerodrome, or professional running courrier, named Pheidippides in 490 BC. Pheidippides’ name would go down in history as the man who carried the news of the victory over the Persians at the battle of Marathon to Athens, running 26.2 miles and promptly dying of exhaustion as he delivered it.

While that famous sequence of events gave us the modern Marathon as the ultimate endurance event, there’s actually historical evidence which suggests that Pheidippides’ true achievement was greater. As Dean recounts in the book, the courier was apparently dispatched to Sparta before the battle to ask the Spartans for their help in the upcoming fight.

Hoping to recreate that earlier epic run – and better understand his own Greek heritage – Karnazes decided to tackle the Spartathlon eating only what Pheidippides would have had available to him. It was a choice which caused him no end of problems.

“I wouldn’t say I’m a masochist, but I look forward to the struggle”

“I ate just the ancient food – olives, cured meat, a kind of beef jerky, and figs. I would run for 6 or 8 hours on training runs eating this food and it was fine. But what I found eating figs for 24 hours caused a lot of problems. You can imagine how much fibre there is in figs…”

Karnazes laughs, but the issue was deadly serious. “I pretty much had diarrhoea, and you know how when you have diarrhoea you don’t feel like running. You’re dehydrated, and that’s kind of where I was at.” It’s here that things started to become dangerous.

“When you’re hallucinating like that and having out of body experiences you’re pretty close to the edge,” he says. “How close, I don’t know. I pushed through it and luckily I’m still here, but you never really know how close… the other story could have been ‘Dean died on this race but you know he had a great life.’” He laughs again, but for a while the risk of ‘doing a Pheidippides’ felt horribly real.

Yet spend any amount of time with Dean and you realise it’s precisely his everyman demeanour, his insistence that he’s not that special, and his easy storytelling style that make him so inspiring. Small wonder he’s the ultra runner that late night chat show hosts want to have on their programs. When asked by David Letterman if there was anything which set him apart physiologically, Karnazes quipped: “Well, I’m not real bright.”

What does undoubtedly make Dean special is his ability to endure (and even enjoy) extreme suffering, and then write about it eloquently. This means his books have inspired people far beyond the world of running.

Ultra Marathon Man is regularly passed around cancer sufferers’ support groups. “I met a woman who was 53 years old,” Dean says. “She had [been diagnosed with] terminal breast cancer when she was 50 but she read my book she was like: ‘I’m not only gonna beat breast cancer, I’m going to become a runner.’ She had the best attitude, she’d had a double mastectomy and she said: ‘Who needed ’em anyway, they just slowed me down.’”

“When you’re hallucinating, and having out of body experiences, you’re pretty close to the edge”

And even for those who don’t actually take up running, Karnazes believes there is something fundamentally inspiring in the act of extreme endurance. “I’ve written four books trying to explain that and I still don’t think I’ve nailed it. But I think there’s a humanity in running and there’s camaraderie in suffering. People that suffer together are tighter. Think about people in wars, military guys, how they form these bonds because of the hardship they went through. I think that running brings that out in people. You can relate to someone in pain.

“Even when you go to these marathons [like London or Boston] there’s non-runners along the course cheering. I’ve talked to some of these folks sometimes like ‘why do you go and watch a marathon? Why are you watching these people just running?’ There’s some power in it. To see another human suffering and struggling and trying to do something, trying to accomplish something, there’s something in us as humans that it kind of draws you in. It kind of moves you.”

As we part ways, and I walk back onto the safe but boring bustle of Regent Street, I reflect that Dean is right. There is something fundamentally compelling about reading tales like Dean’s, hearing stories of human suffering and endurance. After all, why else has Pheidippides’ story survived for 2,500 years?