After Spaced, Shaun of the Dead and Hot Fuzz, it's widely believed that Simon Pegg can do no wrong; appearing as if custom-made to test that contention is the new comedy Run, Fat Boy, Run. Run, Fat Boy, Run gets approving nods from comedy connoisseurs when you mention Pegg's in the lead; mention the presence of David Schwimmer in the director's chair and their expressions change subtly ...

After the success of Friends, Schwimmer could have devoted himself to any number of pastimes -- stick-fighting, model rocketry, leisurely laps in a Scrooge McDuck-style pool of Sacagawea dollar coins. Instead, Schwimmer starred in Duane Hopwood -- which saw Schwimmer playing a barely-functioning alcoholic, as if a brief crawl through the gutter would get the Ross off him -- and here he makes his directorial debut.

Run, Fat Boy, Run stars Pegg as Dennis; five years ago, Dennis left his pregnant girlfriend Libby (Thandie Newton) at the altar. Now, he's living in a basement flat, working as a security guard, occasionally being a good dad to his son Jake (Matthew Fenton) -- and confronted with Libby's relationship with Whit (Hank Azaria), a well-to-do, can-do American. In a moment of weakness -- motivated by pride, or idiocy, or both -- Dennis reacts to Whit's bragging about his upcoming athletic endeavors by saying that he, too, is going to run the London Marathon ... This is, of course, a bad idea. Dennis isn't fat, per se: "I'm not fat, I'm just not fit." In fact, he's extraordinarily unfit -- pale, pudgy and puffing away on an endless chain of cigarettes. But if this is a way to get Libby's respect -- or some attention -- he'll try. Run, Fat Boy, Run comes across as Nick Hornby-lite; it explores the same psychological terrain as About a Boy or Fever Pitch or High Fidelity, where callow and indifferent men find something to believe in despite themselves -- but it doesn't quite have the resonance and depth. The script (originally by Michael Ian Black, although Pegg also receives credit) stays fairly surface, for the most part -- even throwing in a subplot about how Dennis' comedy-relief cousin Gordon (Dylan Moran) has bet the farm on Dennis finishing the marathon in the face of all evidence.

And Pegg is game, certainly -- swanning about in ridiculous shorts, falling over with vim and vigor, more than willing to look foolish for laughs. And there are moments where you get a sense of Dennis as a character separate from Simon Pegg, like when Dennis blurts out why he feels he has to run 26.2 miles even though he looks like he could hardly make it 26.2 feet: "I'm sick of being the 'nearly' man; I'm sick of being scared ..." If you can't sympathize even a little with Dennis's plight in this film, then you're either very principled, very lucky or completely oblivious.

Schwimmer's directorial style mostly involves getting out of the way of the actors; Newton is luminous as ever, Azaria hearty and unlikable. There are a few nice visual moments, mostly when the runner's metaphor of 'hitting the wall' becomes something a bit more concrete for Dennis. In some alternate timestream, Run, Fat Boy, Run is set in New York and stars Adam Sandler or Dane Cook; when Schwimmer managed to get Pegg on board, his good fortune became ours.