Sport on TV: Izzard can barely stand up but his remarkable feat is no joke

Fans of the movie 'Marathon Man' will recall Laurence Olivier's Nazi dentist hissing: "Is it safe?" Now Eddie Izzard is no Lord Larry – although he does a mean James Mason impression – but in Eddie Izzard – Marathon Man, the answer to the question is a resounding "No".

The "actor and comedian who also happens to be a transvestite" (as the narrator, David Tennant, refers to him) promises to run 43 marathons in 51 days for Comic Relief. He's 47. He has never been running before. He's got flat feet. And doctors recommend three weeks' rest after running just one marathon. Even the camera crew's rickshaw breaks down before he does.

It's an astonishing feat of endurance, making David Walliams' inspirational swim across the Channel for the same charity in 2006 look like a mere drop in the ocean.

At least Izzard doesn't try to do it in stilettos. But he regularly puts on fresh trainers. "A change is as good as a rest," he says. "I learned that in high heels, actually." He is given a moment's pause for thought when he wonders if his toenails will drop off and the nurse tells him that he should be more worried about his nipples falling off first.

Dr Mike Loosemore at the Olympic Medical Centre, the British boxing team's doctor, goes very quiet and looks the other way when he hears the plan. Izzard begins his 1,166-mile odyssey around the UK after less than six weeks' training. He is constantly distracted by what's going on around him, his early efforts taking 11 hours as he stops and chats and tweets and devours ice lollies. But there is a darker side to the bravado.

For a while he is joined by the former British distance-runner Bruce Tulloh, who ran 2,876 miles across the United States in 64 days in 1969. "Usually some traumatic experience causes this," Tulloh confides to the camera, and he's right. In addition to the punishment he puts his body through, Izzard suffers emotional torture.

His first destination is Skewen in Wales, where his mother died of cancer when he was six years old. "Why did I go back?" he muses. "Probably to recapture the time before my mum died."

In addition, his publicist, Karon Maskill, makes the point: "He has no fear because he's a transvestite." This supreme physical test is being taken by a man who has never felt comfortable in his own body.

At the end of this absorbing first episode of three, Izzard states: "The brain is telling the body, 'We're gonna run for our life'." He says he has become a machine, but it's a machine that's protecting a fragile and fascinating humanity.

"Maybe I'm just a big kid who's determined to do these adventures that I invent in my head," he says. It's appropriate that he should be running for a charity which helps those who cannot help themselves, especially children.