I should have seen it coming. “Jeremy, you’ve run marathons,” the voice on the phone said. “Yes,” I answered cautiously.

“And don’t you run up mountains or something?” “Well, I do a bit of hill running and I . . .”

“That’s great, we have the perfect assignment for you — next Monday.”

Five days later I’m standing in clapped-out trainers on a pavement in Tulsa, Oklahoma, waiting for a man called Dean Karnazes. Often described as the fittest man on earth, he’s a competitive triathlete, surfer, mountain biker and snowboarder. He has run a marathon to the South Pole, won the Badwater Ultramarathon across 135 miles of Death Valley, and once ran continuously for 81 hours. When he’d finished he had clocked up 350 miles.

Yes, I know what you’re thinking: “Isn’t that enough?” Well, no. Like Forrest Gump, Karnazes can’t stop running, and now he has embarked on his greatest challenge to date: the Endurance 50, sponsored by the North Face, the maker of outdoor equipment. Over 50 consecutive days, he is running 50 separate marathons in all 50 American states.

Tulsa is marathon number 23, and guess what? I’m going to be running it with him, along with 40 others inspired by his extraordinary exploits.

Actually I’m feeling fairly upbeat about the prospect. I run four times a week — six to 10 miles each time, up and down the hills of County Down. Yes, the last time I ran a marathon, in London, I was overtaken by a papier-mâché rhinoceros. But still, this is Karnazes’s 23rd consecutive 26.2-mile race, so just how fast is he going to be? My optimism lasts until the moment Karnazes pulls up in a mini-motorcade. The cars stop, he jumps out, and his groupies burst into spontaneous applause. Oh my sainted aunt: the man looks like he’s made out of metal. His calves bulge as though sacks of King Edward potatoes have been implanted under his skin. He looks fresh, enthusiastic and, above all, strong.

“Sorry to keep you all waiting — are you ready to run a marathon?” he hollers. There are whoops and cheers all round: but not from me, the British bloke with ancient running shoes and white legs, suddenly feeling the effects of jet lag and the bottle of red wine he helped finish last night.

There is a brief round of introductions, the police escorts fire up their Harley-Davidsons, and I check the batteries on my tape recorder (I’m going to interview him on the hoof).

Mile 1 Actually, it feels good to run. The first mile always does. Karnazes sets a steady pace and the world unwinds at a refreshing speed. I begin by asking the marathon man what he had for breakfast.

“I should say something wholesome, shouldn’t I?” he says. “But it was a bowl of Sugar Hoops.” It transpires that keeping up the calorie count is one of his biggest problems. To prepare for the run, Karnazes employed the services of Chris Carmichael, the fitness coach who numbers cyclist Lance Armstrong among his clients.

Carmichael has set a minimum level of 6,000 calories a day, to be eaten between each race, but it isn’t enough. Karnazes has become so hungry he’s been taking pizza deliveries in the middle of each race, rolling up the slices one-handed and eating them without stopping.

“What else can I do?” he asks. “Because my body fat is so low I don’t have the resources in reserve that other people carry.” Apparently his body fat rating is just 4.8%. For a man of his age — 43 — about 20% is considered normal. It’s rare for me, 5ft 11in and 11 stone, to feel flabby, but that’s what Karnazes does to mere mortals.

Mile 3 It’s after the first couple of miles that the niggles start to surface. Mine are in my dodgy left shoulder, the result of a motorcycle accident, and in my right shoe, the sole of which looks to be coming unglued. What are his, I wonder? “Actually, I don’t have any,” he tells me. “My natural running style and the alignment of my joints is good, so I don’t suffer from those major wear-and-tear injuries. I actually feel I’m becoming stronger with every race.”

All right then: what about the minor niggles? Jogger’s nipple, or maybe some unsightly chafing? “No, I wear specialist clothes that wick the sweat away from my body, which cuts down the abrasion,” he says.

In fact the only problems he’s had so far have involved a few run-ins with mother nature. These included a stand-off with a bear in Alaska and trying to outpace an angry porcupine.

Mile 5 By now I’m wishing the bear had intervened more successfully. My trainer is starting to flap around like a clown’s slapstick shoe, and even though I came off the bike 26 years ago my shoulder is beginning to feel as though it happened yesterday.

Karnazes tells me how he began his running career. In his twenties he had a high-powered office job and took little exercise. But when he hit 30 all that changed. “I was out on the town with my friends celebrating my birthday and doing some real bad tequila,” he says, “and I just got this feeling. When the clock struck midnight I told them I was going to run 30 miles. I guess I was having an early midlife crisis.”

While most of us would struggle staggering 30 paces to make the last bus home, Karnazes went straight from the bar and ran the distance without any training. “It wasn’t pretty, but I did it. Within a year I had developed into a serious athlete. I ate the right food and trained very hard over long distances.”

He was still holding down a regular job but running 70-80 miles a week too. It’s only recently, since the publication of his book Ultramarathon Man, that he has run full time.

Mile 8 At this point many marathon competitors start to feel what’s known as the runner’s high. Scientists think it’s caused by anandamide, a naturally produced chemical known as the “bliss molecule” which is also present in chocolate. It produces a tranquil, meditative state, and runners claim their pace becomes very natural and unforced under its influence. Surely this is something Karnazes has experienced? “You bet,” he says. “I’m in the middle of a 50-day runner’s high. It’s the emotional rollercoaster of a lifetime, with plenty of lows as well as highs.”

What was the worst low? “Well, running with a raging head cold in Nebraska wasn’t fun. But the hardest days so far were in Hawaii and Arizona: one marathon in a temperature of 31C, followed by another the next day at 35C.”

The high point? “The end of every race,” he says. “Each day, people new to marathon running have joined me on the course. Seeing them finish has been a huge buzz.”

I nod and force a smile, but I know I won’t be among today’s finishers. My breathing’s fine but my legs feel like lead.

Mile 10 Fifteen minutes later and the call of nature gives me an opportunity to make my excuses. Karnazes is chatting to a goggle-eyed groupie who took up the sport after reading his autobiography. He looks the other way, I slip to the back of the pack and then nip off to find a shady knoll.

Just before I disappear I take a last look at the course. Karnazes is still there, surrounded by his groupies, running on into the distance keeping pace like a metronome.

Karnazes jogged over the New York marathon finishing line on November 5 with a time of three hours — one minute behind the cyclist Lance Armstrong. He had covered a total of 1,310 miles and 2.25m steps after setting off in St Charles, Missouri, 50 days earlier. Now that’s over, he’s running home from New York to San Francisco. Follow his progress on http://rodale. typepad.com/deans_run_home/

Born to run

One of the most surprising aspects of Dean Karnazes’s achievement is that he may not have done himself any long-term damage. “The human body is a remarkably adaptable instrument,” explains Dr Steve Ingham, a physiologist at the English Institute of Sport. “If long-distance running becomes the standard activity, then that’s what the body will become skilled at.”

Karnazes has said he benefits from good natural biomechanics — he runs in a way that doesn’t put joints and muscles under unusual strain.

“I’d expect him to have a very high proportion of slow-twitch muscle fibres in his legs, which process oxygen and energy very efficiently and are far more resistant to fatigue,” says Ingham. “I’d also expect his lungs to have developed a capacity 25-50% greater than a non-runner.”

He says the key to Karnazes’s success is that he ran each leg like a training run — well within his capacity.

Tests of endurance

The Race Across America
RAAM — the annual Race Across America — is the world’s longest continuous bike race. The 3,000-mile event is not broken into stages — contestants sleep when they need to. www.raceacrossamerica.org

The Self-Transcendence 3,100 Mile Race
The longest race on foot is run around a 0.5488-mile course in New York. The runners have to complete 5,649 laps within 51 days. www.srichinmoyraces.org/3100

Badwater Ultramarathon
The world’s hottest race is a non-stop 135-mile run from Death Valley to Mount Whitney, California, in temperatures of up to 55C. www.badwater.com

The Iditarod Trail Sled Dog Race
1,150 miles in sub-zero temperatures, over the mountains of the Alaska ange, create one of the world’s greatest challenges. www.iditarod.com