Last summer Eddie Izzard decided to run around England, Ireland, Scotland and Wales though not necessarily, as Eric Morecambe said, in the right order. The touching and thought provoking programme, Eddie Izzard: Marathon Man, was buried alive quite late at night on BBC3 but, if you missed it, there are two more episodes.

To say he did it for Sports Relief is true up to a point. The real reason is harder to fathom. He had to run 1,000 miles – 43 consecutive marathons – followed by a motorised rickshaw (with the camera crew) and an ice-cream van (with Flake 99s). To see him stumbling along in blinding rain, no cheers, no crowds, no pavements, carrying the sodden flag of the appropriate country, made you want to shout: "Excelsior!"

He was 47 and far more accustomed to six-inch heels than trainers. The director of the Olympic Medical Institute, Professor Greg Whyte, rallying from the initial shock, said: "Being a competitive athlete is fundamentally miserable. What he needs is people around him." You could, of course, say the same of a comedian. In Thurber's story The Day the Dam Broke, one man starts to run and the whole town joins in. So, along the way, Eddie started to attract what I can only describe as sympathisers. In Devizes a man began involuntarily to run beside him, saying: "Let me shake your hand, because I think there's summat wrong wi' you." Seventy-two-year-old Bruce Tulloch, who once ran across America, joined him for a while, wondering why runners run. A couple of wild Welsh ponies, manes flying, galloped up to watch. On the Brecon Beacons, when the motorised rickshaw ran out of puff, his whole camera crew (fluffy caterpillar and all) got out and ran after him.

In Skewen, South Wales, he knocked on a door and said: "My name's Eddie Izzard. I used to live here." His mother, however, died there. "I was six, so I think it's all about that loss of love and my desperation for it. I turned to an audience as a substitute." As a teenager he cycled to Skewen from London. "I keep going back. Probably to recapture the time before my Mum died. It makes you very independent. Very cold and shut off, really. This drive, because of something that disappeared . . . maybe I'm just a big kid still determined to do these adventures I invent in my head."

Running 30 miles a day on blistered and infected feet ("I could lose my toenails," he said. "You could lose your nipples, too," replied Jo, his therapist, heartlessly) gives you plenty of time to think about this and that. If only to stop you thinking about your nails and your nipples. About being a transvestite, for instance. "I'm a transvestite, but I will thump people if they give me a hard time. I don't understand why. But I was given these cards. And I'm honest about it. It's genetic. Not my choice. Just be truthful and get on with it." All punctuated with gasping breaths. "And does the road wind upwards all the way? Yes, to the very end."

Towards the end of the 10th marathon, he collapsed. When they told him there were three switchback miles to go, he said: "I do that then." His therapist begged him to walk, but he did it running and, to celebrate, freewheeled downhill into Builth Wells (pop. approx 2,000) and a rousing civic reception. "I like making a little bit of a rumpus, a little bit of a circus coming to town, because I grew up in Bexhill, and not much happens in Bexhill." Even less, I suspect, in Builth Wells, but when it does they make the most of it. Twitter had gone ahead like a carrier pigeon and the street was packed with cheering people, boys pulling faces at the camera, and the obligatory dog. Eddie's unadvertised gig seemed to be collecting an audience as it went along, as a boat collects barnacles.

The sunset sky glowed rosy. Perhaps it wouldn't rain tomorrow. He had run 277 miles in 11 days. There were only 800 or so more miles to go. He said: "I will finish. By hook or by crook, I Will Finish."

And you knew that, short of a thunderbolt, he would. If you are going to run you must – so to speak – be driven.