Generalizing about "British humor" gets you about as far as saying all Westerns are created equal, or that one film adaptation of a Broadway musical is as good or bad as another. I will say this, however: One of the problems with the new comedy "Run, Fat Boy, Run" is that it's not English enough, even though its antagonist is a thoroughly detestable American go-getter.

The project represents an uncertain Britification of a script by Chicago native Michael Ian Black, and the result finds itself stranded somewhere out in the Atlantic, mid-crossing. Black's rewrite man was Simon Pegg, who stars. He's a wonderful comic actor ("Shaun of the Dead," "Hot Fuzz"), and he periodically bails this contrivance out of movie comedy jail.

Pegg plays Dennis, a mildly schlubby commitment-phobic Londoner who, at the altar, ditches his pregnant fiancee (Thandie Newton, looking grave) out of sheer panic and self-loathing. Five years later they're sharing custody of their son but nothing else. Newton's Libby is now spending time with a smug American fitness freak played by Hank Azaria. Goaded by Azaria's character, Dennis enters the London marathon to prove his mettle against his romantic rival and to win back Libby's heart. And there's your story.

And where are the jokes? Too much of "Run, Fat Boy, Run," directed by David Schwimmer, provokes the question. No one should expect Pegg's projects to stick with sharp-eyed satire in the "Shaun" and "Fuzz" vein. He's fully capable of becoming a major comic leading man, and in England, he's already there, having navigated a variety of genres. He has a marvelous face for comedy, and like Nicolas Cage or Gene Wilder at their best, Pegg can sprint from deadpan underreaction to pop-eyed overreaction in 0.8 seconds flat. But "Run, Fat Boy, Run" is strictly routine on the page. By the big race the romantic comedy cliches and the sports-movie cliches prove insurmountable. And talk about product placement! Either the film should've been titled "Run, Slightly Flabby Boy, Run" (truth in advertising, given Pegg's physique) or "Sponsor, Nike, Sponsor" (just plain advertising).