Pudgy, yes, but charming at times

Sometimes, Simon Pegg betrays too much suppleness, physically and otherwise, to convince you that he’s packing a paunch as Dennis the lovelorn, chain-smoking slacker in “Run, Fat Boy, Run.” But he’s such a good actor that he makes you believe anything -- anything, that is, except the notion that any sentient male Earthling, no matter how stupid or scared, could leave a pregnant woman looking like Thandie Newton standing at the altar.

This happens at the beginning of the movie, firmly establishing Dennis as being too pathetic to deserve even his rent-a-cop gig in a London lingerie shop. He’s still pining for Libby (Newton) and enjoying his bonding weekends with their 5-year-old son. But there’s another man in Libby’s life: a Type-A, buffed-up American sharpie named Whit (Hank Azaria), who’s primed to seal the deal with Libby that Dennis abandoned -- right after he competes in the city’s biggest marathon. To everyone’s surprise, Dennis says he’s also running in the marathon, even though he can barely trot across the street without hacking.

Snarkiness and sentiment are in constant battle for supremacy throughout “Run, Fat Boy, Run” with no chance of a comfortable draw. Maybe that’s because David Schwimmer (yes, the same guy who played Ross on “Friends”) makes his directorial debut with this script by Pegg and Michael Ian Black, who has been a snide talking head on those “I Love the ‘70s/’80s/’90s” VH-1 shows. Despite an all-out effort by an appealing cast (with props to Dylan Moran, hilarious as Dennis’ lowlife best buddy), the movie hits the proverbial wall toward the end through needless over-exertion of its sentimental side.

Pegg, however, still rocks as a comedic presence, able to juggle groggy and goofy mood swings without losing his grounding in credible human behavior. And Schwimmer shows enough promise behind the camera to deserve another chance. Couldn’t he try again in the tri-state area, though?

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“Run, Fat Boy, Run.” MPAA rating: PG-13 for some rude and sexual humor, nudity, language and smoking. Running time: 1 hour, 37 minutes. In general release.