Iron Man
by Paddy Doyle
Blake Publishing, £15.99

Paddy Doyle, the world's fittest man, opens the door to the house he shares with Samantha, his fiancee and Carla, his rottweiler."She won't hurt you," Doyle says. Rottweiler owners always say that but, in this case, it appears to be true. After opening her jaws into a chasmic yawn, and exposing a menacing set of teeth, she settles back down into a rumbling slumber.

Doyle has always kept "rotties". The breed goes with his image. There is a photograph of him and Carla eyeballing each other on the back cover of his ghosted autobiography, Iron Man: the true story of Britain's strongest, fastest, hardest man. Strongest? He is 38, and weighs just over 12 and a half stone. Younger, bigger weightlifters can hoist heavier bars than he can. Fastest? Sprinters can cover 100 yards faster. But that is not the point. Doyle is an endurance athlete who would never be content with, say, completing the London marathon in a good time. He would do it with a 40lb pack on his back.

He was carrying 45lb when he completed a gruelling 25-mile circuit of the Peak District in eight hours, 10 minutes last month. "In those situations, I focus my mind on the finishing line and imagine that I'm there. I'm told that my eyes were bulging like a bulldog's bollocks. Spectators were backing away."

It turned out to be the 124th time he had broken a world record, which was itself a record. After due consideration, the Record Holders' Association declared him to be the fittest man on the planet. For the time being, at least.

One of the drawbacks about being a world record-breaker for "five-hour one-arm press-ups" or "one-hour squat thrusts" is that someone in Texas or Tasmania might be breaking your record at any moment. Since 1990, Doyle has had 31 entries and re-entries in the Guinness Book of Records and 10 are still standing.

Upstairs, a spare bedroom is crammed with his trophies. Shelves groan under the weight of his cups and walls are festooned with belts from his amateur boxing days. When I suggest that 38 might be getting on a bit for an endurance athlete, his bruiser's features soften into a grin. "We mature like fine wine," he says. "The sporting cycle has changed. Twenty years ago, I reckon that somebody doing what I do would have peaked at 28. Now it's more like 38-plus. Why? Because we have more scientific training methods and a better diet."

For someone whose idea of keeping in trim between tournaments is a six-mile run every two days, preceded by 500 or so press-ups and an equivalent number of sit-ups, Doyle's food intake is surprisingly slight. Breakfast is a couple of slices of wholemeal bread with low-fat spread. "If I'm in full training, I have these," he says, returning from the kitchen with three rattling canisters.

"Pushing yourself to the limit in the gym can run down your immune system and leave you open to infections, so I take a multivitamin every day. I also have folic acid because it's good for the heart, and Omega 3 fish oil for the joints and digestive system."

Lunch might be a wholemeal tuna sandwich or a baked potato and baked beans. He juices his fruit rather than eat it whole. Dinner is fish or chicken with vegetables and potatoes. No chips or steak? "Once a week," he replies. "A bit of red meat does you no harm, and I burn off the fat from the chips pretty quickly. We also treat ourselves to a Chinese take-away now and again." Does he have a glass of wine with it? "Two glasses of red once a week and a few pints on Friday night."

Doyle was brought up in the shadow of Spaghetti Junction in Birmingham by his father, one of many Irish labourers who helped to build it. His parents split up when he was four. Not surprisingly, perhaps, he grew up bitter and resentful. At the same time, he was bursting with an energy and strength beyond his years. "I think I must have inherited it from my grandfather on my mother's side," he muses. "He boxed for the British army."

Always sporty and competitive, Doyle took up martial arts aged seven, and became a junior judo champion. Wherever his power and anger came from, it was a combustible mixture. When it wasn't being channelled into competitive sports, it was finding an outlet in other ways. Doyle had his first brush with the law at nine, and his youth was scarred by innumerable brawls in back streets, pubs and clubs. But he turned his life round and has never looked back.

"I know where a lot of today's young headbangers are coming from," he says. "They listen to me when I go into schools to talk to them. I tell them that sport turned my life around by chanelling my aggression."

That may be so, but youngsters less than half his age would struggle to match his physical feats, even if they followed a regime of monastic discipline and gymnastic dedication. Few human beings are blessed with Doyle's combination of a high pain threshold, low pulse rate and phenomenal cardiac output.

After breaking his own record for "burpees" - squat thrusts beginning with and reverting to a standing position - his friend and osteopath, John Williams, attached a heart monitor to his barrel chest. "I was amazed at what it recorded," he recalls. "He'd done 3,000 burpees in boiling hot sun, yet the rate was low and constant as though he'd done no exercise at all."

But couldn't constantly pushing himself to the limit be building up problems in later life? "Yes, it can lead to arthritis," Williams concedes. "But I don't think Paddy will suffer more than anyone else. As he's grown older, he has learned how to take the right advice. He wouldn't, for instance, do what too many professional footballers have done and have injections to get them through the pain barrier."

Samantha, 33, finished the Peak District circuit an hour behind her fiance. "It gave me an insight into what he does," she says. "My knees were swollen and aching, and I wasn't carrying any weights."

He may be obsessive about his training but Doyle says he is a different man when he is not competing. "I've found the right woman at last," he says. "Sometimes I'm so laid back at home that I almost fall over." Rather like his rottweiler.

· Iron Man by Paddy Doyle is published by Blake Publishing, priced £15.99