Why in hell is Queens-born David Schwimmer making his directing debut with a British farce? Damned if I know. But the Friends star has a welcome eye for laughs that sneak in around familiar corners. It’s also a big comic boost that Simon Pegg, the offbeat star of Shaun of the Dead and Hot Fuzz, plays Dennis, a security guard in a lingerie shop. Five years earlier, dumbass Dennis ditched his pregnant fiancée, Libby (gorgeous Thandie Newton), at the altar. Now she’s about to marry Whit (Hank Azaria), a rich American. Desperate to win Libby back, Dennis competes against Whit in a London charity marathon to prove he can man up. “I’m not fat,” Dennis insists, “I’m unfit.” So’s the plot, which sparks only when Schwimmer spins knockabout variations on it. The funniest scenes are between Pegg and Dylan Moran as Dennis’ swanning BFF, Gordon, who always seems to have his bare ass showing. Even the Farrellys might gag when a lanced blister on Dennis’ foot sprays its infected goo smack in Gordon’s face. “That’s the second-most-disgusting fluid I’ve ever had in my eye,” says Gordon. You get my drift. And even if you don’t, Run, Fat Boy, Run stays out of sitcom quicksand long enough to make you think that Schwimmer has a knack for this comedy-directing thing.