Sport on TV: Davina insists on Big Sister role but Jimmy is just car-crash telly

One of the joys of Sport Relief is watching pampered celebrities being taken out of their protective cocoon to perform challenges which strip away the make-up and leave them grimacing in pain.

After inflicting 'Big Brother' on the nation for a decade, it was cheering to hear Davina McCall say: "I like to do something that gets my life in perspective." Cycling alone through the Highlands in the dark in -15C seems a suitable punishment, a cycle of despair that almost makes up for all the show-offs making fools of themselves in an artificial environment. And it almost makes up for 'Big Brother' too.

To fully absolve herself, she would have had to cycle all 1,000 miles in David Walliams' non-stop four-day challenge, Million Pound Bike Ride (BBC1, Thursday). But she had six accomplices – plus Jimmy Carr, whose surname is apt. The comedian only joined them for a day in the middle of his latest tour, and even his audience was telling him to get on his bike. He put his relay team-mates hopelessly behind the clock, and opined: "I think training is almost as bad as drugs in sport. It's cheating." The viewer might have felt cheated, not only on behalf of the charitable causes that he effectively shunned, but because there was not enough pain inflicted on those cheeky, cherubic features.

It might sound extremely small-minded, but some of these altruistic endeavours are starting to resemble self-serving publicity stunts. Walliams is off the hook in this regard, having swum the Channel so courageously in the early days of Sport Relief. So is everybody's favourite transvestite, who in the last episode of Eddie Izzard – Marathon Man (BBC3, Thursday), finished his 43 marathons in 51 days by running one a day for the last 10 days despite suffering ankle ligament trouble and, finally, chest pains.

His 1,166-mile epic was largely solitary, being splashed and squashed by lorries. All seven of his camera crew's rickshaws broke down before he did, and there were about a hundred people waiting at the tape in Trafalgar Square. There should have been a million. Like Walliams in the Channel, such was the scale of his undertaking, physically and geographically, that he was by necessity mostly alone. Which was all the sadder because his impregnable sense of humour was a perfect foil for the searing personal insights he stumbled upon.

Izzard has set the bar so high that most Sport Relief feats will now look lame in comparison. Perhaps it's time for the real sports stars to stop trying to be comedians and up the ante – though they can always claim their insurers won't allow it. Steve Cram joined Eddie but because he was carrying an injury, he did so on a bike. Unlike our hero, he couldn't giggle through the niggle. The only thing he ran out of was excuses.