Boy runs out on pregnant girl on wedding day. Girl gives birth to son. Five years pass; girl is now ready to marry rich American. Boy then realises girl and son are the only reasons to turn his sad existence of a life around, and is desperate to win her back.

What's a loveable loser boy with a fat gut to do? Duh, run a marathon to win the girl, of course.

It's silly, formulaic and never really threatens the ribs, but as far as Brit-coms go, Run, Fat Boy, Run is a cheery mix of juvenility and grown-up relationship angst, which runs smoothly enough to be worth the watch.

Friends star David Schwimmer marks his movie directorial debut with an eye for easy laughs that sneak in around familiar corners, italicising the jokes and adding exclamation points with sufficient confidence.

He turns what could have been a determinedly-creaky cliched underdog plot into a likeable oddball project, thanks to a wicked cast and polished script by leading man Simon Pegg.

Anglophiles will gush over Britain's funnyman-of-the-moment Pegg's (of hilarious Shaun of the Dead and Hot Fuzz fame) impeccable comic timing and acting chops. His every action and reaction is winningly human as he handles the romantic-lead stuff as skilfully as the pratfalls.

Sprightly supporting turns by Thandie Newton, Hank Azaria and British comic Dylan Moran (in a show-stealing infected-blister scene with Pegg that will challenge even the Farrelly brothers' gag reflexes) must also be commended.

This feel-good comedy may be blatantly unrealistic, but like so many dopey farces before it, it is as intuitively likeable as the sad loser at its centre.

Yes, the laughs slow to a trickle at times, but whenever Moran and Pegg are on screen, the movie feels less like a never-ending marathon and more like a fun Sunday run. - TODAY/sh