Simon Pegg is once again the quintessential UK slacker in "Run Fat Boy Run."

It's a rehash of the role he perfected gloriously in "Shaun of the Dead," - shiftless boyfriend who tries to win back exasperated/neglected girlfriend.

Only without any of that movie's energy, wit or genre-bending inventiveness. Instead, we get the sort of flat-footed studio film that takes the basic ingredients of an underground hit, drains them of personality and style, and spits them out as a situation comedy.

Call it "Shaun of the Dud."

Pegg is his old pleasant self here, and he does his best, but there are fundamental problems, starting with the obvious fact that somebody is badly miscast.

There is no way to explain why a potbellied fellow with a fish-belly complexion and limited prospects (he's a security guard) would end up with a dish like Thandie Newton, let alone ditch her (pregnant) at the altar.

Nor is there any way to explain why Newton's character would ever take him back. "Fat Boy" offers an array of hack devices in an attempt to make it credible - starting with their son, the stock cute kid who wisecracks like an adult, and who obviously hates mom's new beau (Hank Azaria).

And why not? He is as rich and shallow as Pegg is fat, poor and soulful. I should pause here to remind young men that this is a comic fantasy. You should know that if you're dating someone even remotely like Thandie Newton, you should expect her to take rich and shallow over poor and fat every time. I'm not saying it's right. I'm just saying.

The movie is only interesting so long as Azaria's character looms as a legitimate alternative to the titular fat boy. But "Fat Boy" wants Pegg's rival to become caricature. What's his pariah-of-the-moment job? If you guessed hedge-fund manager, you could be president of script development at New Line.

The ridiculous plot has Pegg proving his mettle by beating fitness-freak Azaria in a marathon, during which he becomes a pop hero by fighting on through a leg injury.

I empathize. This felt like 26 miles of bad road. *

Produced by Sarah Curtis and Robert Jones, directed by David Schwimmer, written by Simon Pegg and Michael Ian Black, music by Alex Wurman, distributed by New Line Cinema.