Simon Pegg stars as a slob who decides to enter a marathon to win back his ex-girlfriend in Run

The only time eternal quitter Dennis (Simon Pegg) has gone the distance was when he did a panicked high-speed sprint in formal dress, leaving his stunned (and pregnant) fiancee at the altar. Five years on, he still sees Libby (Thandie Newton) to pick up their son for afternoons in the park, but that’s about the limit of the apathetic Londoner’s capacity for commitment.

While he’s regretful for ditching her, he explains, “I thought spoiling your day would be better than ruining your life.” Edging into his 40s, he’s sporting a middle-aged paunch, working a dead-end job as a security officer in a lingerie boutique, and is behind on his rent in a basement apartment that resembles a Dumpster. Tree sloths make more effort.

When Libby introduces her rich, accomplished and fit new American boyfriend, Whit (Hank Azaria), Dennis’ misgivings over abandoning her boil over. Radiating Mr. Wonderful self-satisfaction, Whit mentions that he’s running the London Marathon in three weeks. Dennis, determined to impress Libby, casually announces that he’ll be competing, too. The runaway bridegroom then embarks on a crash course in self-improvement, complicated by the fact that he’s winded after a one-block sprint.

While the setup promises a routine romantic comedy with training pratfalls, the film is better than expected. It’s directed with warmth, humor and intelligence by David Schwimmer, who has learned a thing or two about comedy from a decade on “Friends.” Pegg, a cult hero for writing and starring in “Shaun of the Dead” and “Hot Fuzz,” once again juggles scripting and acting duties effortlessly.

“Run Fatboy Run” is a novel romcom told from the male point of view. Pegg’s Dennis isn’t merely an inept comic underdog – he’s a credible, even poignant character who gradually wins us to his side.

As in his earlier films, Pegg effectively works dramatic moments and honest sentiment into the design while surrounding himself with a strong comic support network. Dylan Moran is irrepressibly droll as Dennis’ best buddy Gordon, a sleepy-eyed slacker who lances a blister on his friend’s foot, receives a face full of yellowish goo and remarks, “That’s the second most disgusting liquid I’ve ever had in my eye.”

Dennis’ Buddha-shaped Asian landlord, played by Harish Patel, bikes along as he jogs, encouraging him to keep up the pace with stinging slaps of a spatula. He believes in Dennis’ capacity for spiritual reincarnation – given the right sort of encouragement. The beauty of the film is that we do, too.