Simon Pegg has slaughtered zombies in Shaun of the Dead and taken down a kill-happy cult in Hot Fuzz. But as the lead loser in Run Fat Boy Run, he faces a threat far more evil: the American romantic comedy. The film may look British and sound British, set in London and boasting U.K. stars who get off a cheeky one-liner or two, the driest and silliest most likely courtesy of Pegg’s tune-up of the script. Once the film declares its central conceit, however—out-of-shape schlub who left his pregnant bride at the altar five years back enters a marathon to “prove” he loves her—the movie’s American roots begin to show. With first-time feature director David Schwimmer shooting a screenplay originally credited solely to Stella’s Michael Ian Black, the film gambles on alienating Pegg fans, painting by every rom-com number save for doggie reaction shots: Dennis (Pegg), a security guard at a lingerie store, is still in love with Libby (radiant placeholder Thandie Newton), a torch continually fanned because he shares custody of their son, Jake (Matthew Fenton). When he picks up Jake one day and is unexpectedly introduced to Libby’s new boyfriend—a buff hedge-fund manager named Whit (Hank Azaria)—it’s clear he needs to step up his game, which has mostly comprised screwing up playdates, whining to his Kramer-like best friend, Gordon (Shaun’s Dylan Moran), and arguing with his landlord, Mr. Ghoshdashtidar (Harish Patel), every time he forgets his keys. Dennis finds out that Whit is running in a big London marathon—“Why would you do that?” he asks while puffing on a cig—so, naturally, the rent-a-cop who can barely catch shoplifting transvestites decides to run, too, with Gordon and Ghoshdashtidar as his wacky training coaches. Although you can see the finish line from the starting shot, Pegg elevates the movie from eye-rolling to forgivably genial with his ace delivery and genuinely entertaining pratfalls. And though it’s not quite Apatow-blue, the script cuts its saccharine quality with, say, frequent (and very funny) jokes about what Dennis would call “the scrotal zone” or daddy-son heart-to-hearts that begin, “Listen, I’m really sorry about getting you arrested the other day.” In the end, date-movie suckers may shed a tear—right alongside Pegg boosters, though they’ll be crying for altogether different reasons.