As a movie with a title that reminded me of my experiences at Little Athletics, I can't say I was sprinting to see Run, Fat Boy, Run.

Happily, this British comedy proves less a gruelling slog than a fun run.

Five years ago, Dennis did a runner on his beautiful, pregnant bride-to-be Libby as she stood at the altar. It's a mistake that's haunted Dennis, who's now overweight, out-of-shape and works as a security guard.

His only link to Libby is through their son. But when Libby falls in love with Whit, a smooth American investment type who runs marathons, Dennis's competitive spirit is suddenly brought out. He figures that if he can beat Whit in the race, he might reclaim Libby's love and respect.

There's no getting around it, Run, Fat Boy, Run sounds horribly contrived. And the movie doesn't get off to a great start: Dennis is such a prat that it's hard to like him or find him very amusing.

That leaves it to Simon Pegg – the British comic actor and writer best known for Shaun Of The Dead and Hot Fuzz – to make this character grow throughout the movie. And he really does grow on us, as his quest becomes funnier and more involving.

Pegg gets good support from the charming Thandie Newton as Libby and a suitably unctuous Hank Azaria as Whit. But it's Dylan Moran, best known for the TV series Black Books, who steals every scene as Dennis's caustic, cynical best friend.

Behind the camera, another TV comedian, David Schwimmer of Friends, directs in unfussy fashion that makes good use of London locations.

Run, Fat Boy, Run comes with predictable elements - perfect Whit has a dark side to make Dennis more gallant, and there are the usual Brit-comedy bits featuring foul-mouthed old people and nude bottoms. But there are a lotta laughs, regardless, and some howlingly funny moments involving blisters, mannequins and Dennis's eventual, unlikely triumph.

Run, Fat Boy, Run might not be the movie title of the year, but it's certainly a breezily enjoyable slice of silliness and rates three stars. Run, Fat Boy, Run is in cinemas now.