There comes a time in every successful comedian's life when a film producer suggests they would make a great romantic lead. It takes humility and a massive indifference to money to laugh such offers out of court.

Like so many before him, Simon Pegg, the genius behind Spaced, Shaun of the Dead and Hot Fuzz, has decided he's got what it takes to win the heart of a drop-dead gorgeous woman on screen. For the first hour of this rom-com, you may simply want to run a mile.

Pegg's character, Dennis Doyle, left his pregnant wife, Libby (Thandie Newton), at the altar - he was scared he wasn't good enough for her. Five years later, when she takes up with a super-fit American smoothie (Hank Azaria), this lazy, chain-smoking under-achiever is spurred into action. With the help of his feckless Irish best friend (Dylan Moran) and cheery Asian landlord (Harish Patel), Doyle starts training for the Nike River Run. He wants to regain Libby's respect, and show his son that running away from problems doesn't solve them.

It's easy to blame Friends' cutie David Schwimmer, here trying his hand at directing, for the film's awkward tone and non-atmosphere. In truth, it's the script (written by the American Michael Ian Black and rejigged by Pegg) that makes you blush. The East End locations may be authentic but the characters are out of this world. One moment of hilarity involves an old woman saying "cock!" Later the landlord says a rude word. Blimey. Pensioners and Asians talking like thirtysomething white males. Who'd have thought it? The jokes come thick and slow, Newton has nothing to do except look great and even Pegg's adorable face-scrunching palls.

It's not all bad. The second half picks up belated momentum, thanks to some neatly juvenile set-pieces (an exploding blister; a girlish fist fight). Still, it's hard not to read Run, Fat Boy, Run as an indication that Pegg is in crisis. While Shaun of the Dead and Hot Fuzz poked fun at cinematic conventions, Run serves up its clichés straight. Let us take hope from the career of Peter Sellers and pray that Pegg, having produced his equivalent of The Millionairess, can start on something as good as Dr Strangelove. The nightmare scenario: that, aged 37, our hero is already running on empty.