By common consent the marathon is a unique sporting challenge, even for sportsmen . . . even for sportsmen of the highest calibre.

Olympic rowing champions Matthew Pinsent and James Cracknell know the pain of preparing to reach the peak of human physical achievement, as do ex-British international 400m men Jamie Baulch and Iwan Thomas, while Dwight Yorke climed to the summit of international club football when he won the European Cup with Manchester United.

But all five agreed today that there's nothing quite like training to run 26.2 miles.

"It's just so far," said Pinsent, who made his marathon debut in 2006. "We occasionally rowed in a half-hour race in Germany and we always thought that was a bit mental, but four hours is something different."

That's the target for Pinsent this time as he seeks to raise money for Alzheimer's UK. In 2006, he finished in 4 hours 8 minutes. "That eight minutes has been bothering me for five years," he admitted. "My aim is to get under four hours this time. It's just I'm five years older and five years stupider."

For Cracknell, who came home in under three hours that year, this year's race represents a different kind of challenge. "I got knocked off my bike in America last summer and the frontal lobe of my brain took most of the impact," he explained.

"It's not like a broken limb which you know will heal. The brain, even mine, is very complicated. I've had to learn how to cope, and how to come to terms with it. I broke my foot in the accident too, so I haven't done any running till the last three weeks.

"I'm sure I'll go off with great ambitions, but I know I'll have to revise them."

That's a scenario Thomas knows only too well. The former European and Commonwealth champion admits he went off "way too fast" on his debut last year and "literally died" in the second half of the race. "I won't do that again, but I'll still be behind him," said Thomas gesturing at Baulch, his former training partner and Welsh team-mate.

"We are great mates but we're competitors too and I don't want to be close to him because we'll start competing."

As for Baulch, the one-time world indoor champion explained how the chance remark of a Waitrose cashier persuaded him to enter the race. "I was going through the tills when this woman said, ‘You're not fat at all.' I said, ‘What do you mean?' and she explained she'd seen Iwan on TV saying I was fat. So, I thought, ‘Right, I'm going to beat his time.'"

Thomas finished in 3:58 last year, so Baulch has cheekily targeted 3:57. "This isn't a mid-life crisis but if I go under four hours I'll go out on Monday and buy a Harley," Thomas responded.

As for Yorke, the former striker is taking it all much more seriously and has his eyes on breaking the three hour barrier to raise money for the Vision charity. "As soon as I committed myself to doing it, I went out on the roads and started training," he said. "At the beginning I didn't feel I would be up to it because it's completely different to the training I'm used to.

"But in the last month I've become a lot more confident. Vision is important to me because of my son Harvey," he added. "I felt it was a good idea to raise awareness. And I also like a bit of a challenge."

Baulch, the other debutant, in the line-up, admitted he's trained hard too, getting up to 24 miles, while Thomas's focus has been completing a half marathon in a wheelchair a few weeks ago.

"I've only run in the last few weeks," he said. "As long as Jamie doesn't go off crazy like he did in the 400s, he'll be OK."

"I do want to go under four hours," joked Baulch. "And yeah, it'd be great to beat Iwan as well. He always pretends he's the underdog, but we'll have to see."

As for the pain they should expect, Pinsent had this advice for the debutants. "Rowing pain is intense, a burn. In the marathon it's less intense but it lasts longer, it's more of an ache."

"People think because we were sportsmen, we ought to be able to do it easily," agreed Thomas. "But we were sprinters. I never ran more than 20 minutes in winter training. The crowd are great in London, except last year, when I was struggling along at mile 21, and this one guy shouted, ‘What happened to you? You used to be an awesome athlete.'

"People think if you're a runner you can run any distance."

"I think I'll just put my head down and run," said Yorke. "That's the key to it."