Ask any marathon runner and he or she will tell you: The finish line is less important than — and only attainable with — a full 26.2 miles of strong mind and smooth form.

In many ways, it’s a philosophy of endurance that runs counter to the way Hollywood thinks about the typical romantic comedy. Stumbling through sloppy premises and increasingly contrived conflicts, most modern comedies concern themselves only with the foregone conclusion, making a mad dash for happily-ever-after. Thankfully, it is with a marathoner’s mentality that “Run Fat Boy Run” arrives in theaters today with a story that is in part about runners and second chances, but mostly about a lazy, middle-age man gradually jolted from his waking coma. It stars a chubby but cute Simon Pegg, who here makes a more subdued turn after battling zombies in “Shaun of the Dead” and busting bad guys in “Hot Fuzz.” In “Fat Boy,” Mr. Pegg plays it straight as Dennis, a sprinting skinny boy who ditches the pregnant Libby (Thandie Newton) on what is supposed to be their wedding day before resurfacing years later sporting a notably heftier frame.

Dennis’s rolls of fat and his job as a rent-a-cop at a clothing boutique in downtown London denote a man who is not quite riding high on life. Since he ran away from his wedding, things have gone down the drain. He lives in a dumpy basement apartment and can’t convince Libby to afford him a second chance — not that he deserves it. But when Dennis discovers that Libby is now dating Whit (Hank Azaria), a wealthy banker and a committed marathon runner, he decides to sign up for the London marathon. It’s three weeks away.

What’s more likely: Dennis completing a marathon, or Libby taking him back? Unsurprisingly, the two questions are linked. Dennis commits himself to the task with the same degree of misguided passion that has led so many men to do stupid things in the hope of winning attention when a sincere conversation might have done the trick.

After seeing the film at its premiere at last year’s Toronto Film Festival, several commentators dismissed David Schwimmer’s directorial debut, dubbing the lighthearted story about sports and romance unremarkable. But what I find refreshing about “Run Fat Boy Run” is the way Mr. Schwimmer — not to mention Mr. Pegg and Ms. Newton — sells us on the emotions that hinge on all this silliness. One could build a library out of films that were ambitious in their desire to please but devoid of emotion, and “Run Fat Boy Run” is in some way the exact opposite. It feels more than it thinks, and baits our hearts even as it’s being dismissed by the logic centers in our brains.

In purely emotional terms, the film is far more complex than some will expect. Dennis is hardly a flawless hero. He spends part of the film flagellating himself for running away from the perfect woman on the biggest day of her life, and another part drinking himself into a stupor at the thought of Libby in bed with Whit. As a father, he disappoints his son more than once. His preparation for the marathon isn’t shown here as a flabs-to-riches miracle, but as a gradual and meaningful mental victory: the first time he channels his self-loathing into something positive (similar, in fact, to Mr. Pegg’s character in “Shaun of the Dead,” who bettered himself and won back his love by butchering zombies).

Libby and Whit also defy expectations, the latter especially, because he isn’t the two-faced, snarling new boyfriend of Hollywood lore, but a nice guy in his own right who extends an olive branch to Dennis even as he moves in on fatty’s wife and child.

There’s no denying that “Run Fat Boy Run” turns on a well-worn premise, but is the simplicity in itself a bad thing? Once upon a time, there were competent, comfortable projects — not just comedies, but thrillers, action films, and even superhero adventures — that didn’t break new ground but managed to color inside the lines with a few unexpected shades.

For that matter, we used to see more performers such as Mr. Pegg breathing life into everyday characters as average-looking people, silly but still funny, earnest without being condescending. Dennis is an endearingly damaged hero, a man who sees in his family a reason to improve himself.

ssnyder@nysun.com