Simon Pegg spoofed the conventions of zombie movies and buddy-cop actioners so well in “Shawn of the Dead” and “Hot Fuzz” that it’s a surprise to find him in a romantic comedy that embraces the cliches of the genre instead of sending them up as they deserve. “Run, Fat Boy, Run” just follows the standard rules of the game, and in doing so stumbles and falls. That’s probably not a great surprise, seeing that it’s directed by David Schwimmer, the erstwhile star of “Friends,” through whose veins the sitcom spirit seems to flow.

The fat boy of the title (though he protests that he’s “not fat, just unfit”) is Dennis (Pegg), whom we first meet at a wedding where he proves a nervous runaway groom, leaving his pregnant girlfriend Libby (Thandie Newton) at the altar. Five years later, he’s an inept security guard at a London lingerie shop, keeping up a good relationship with his darling son Jake (Matthew Festoon) and hoping that one day Libby, whom he still loves, will forgive him and take him back. Unfortunately for him, she’s being wooed by a far smoother fellow, Yank abroad Whit (Hank Azaria), a well-to-do hedge fund manager whose closeness to both her and Jake sends Dennis into a near-rage, though he tries to hide his distaste for the guy. The plot kicks in when Dennis decides to run in a charity marathon along the Thames for which the health-conscious Whit has already signed on—to prove to Libby that he’s a changed man, a responsible guy who can actually follow through on something to the end.

What follows is predictable stuff as Dennis engages in slapstick training under the watchful eyes of his pal (and Libby’s brother) Gordon (Dylan Moran), an inveterate gambler who puts his welfare on the line by betting his poker cronies that Dennis will finish the race, and his obese Indian landlord Mr. Ghoshdashtidar (Harish Patel, in a crude stereotype). There’s lots of bodily-function humor here, as well as jokes-to-make-you-gag involving a huge, puss-filled blister, too-short-shorts and naked male derrieres. There’s far less wit, however: this is more Benny Hill than Ealing Studios.

Everything culminates, of course, in the race itself, a really drawn-out affair in which Whit shows his true colors and Dennis demonstrates such grit that things between him and Libby turn out just as you knew they would. One might expect this finale to turn into a satire of such tortoise-and-the-hare nonsense, but not so. It’s actually played for sentiment; even Alex Wurman’s score strives for a “Chariots of Fire” triumphal tone. The result is ridiculous, but not intentionally so.

As you can see, this is pretty much by-the-numbers stuff, cobbled together with a surprising lack of imagination by Pegg and Michael Ian Black; and its success really depends on the charm of its leading man. But that proves a commodity which, in Pegg’s case, proves to be not only in short supply but positively out of stock. In his hands Dennis is basically a selfish, sarcastic jerk, a really unappetizing character. But he’s not alone. We’re supposed to see Newton as a perfect mother, but it’s hard to sympathize much with a woman so careless about raising her child that she allows him to run away at one point and, at another, doesn’t even stop him from fiddling with a hospital bed someone’s lying on. (And then, when that person yells at the kid, it’s supposed to show us he’s a horrible person.) And Azaria’s played this kind of “other man” role before, and isn’t much better at it this time around.

As for the supporting players, we’re meant to find Moran hilariously egocentric, rude and tart-tongued; but while he’s all those things, there’s nothing hilarious about him. And Patel is a caricature so broad he’d make even the most remotely sensitive person wince. Festoon is cute but, ultimately, pretty annoying.

London looks pretty enough in Richard Greatrex’s cinematography, but that’s just window dressing in a routine romantic comedy that’s even less intelligent than most and might have been set anywhere with no appreciable loss. “Run Fat Boy Run” is a picture to steer away from, not hasten to.