Jillian Bell goes the distance in 'Brittany Runs a Marathon' breakout role

“Brittany Runs a Marathon” - ★ ★ ★

Getting in shape and putting your life together usually, at the movies, passes in a montage. But for many, the non-Rocky legions of us, such transformations require more than inspiring theme music. And for the Brittany (Jillian Bell) of “Brittany Runs a Marathon,” that's especially true.

Brittany is an aimless 20-something in New York who's startled when her attempt to pry an Adderall prescription from a doctor turns into a recommendation to change her hard-drinking lifestyle and lose weight.

Brittany gets over her initial offense and, after perusing Instagram posts that only increase her self-loathing, decides to go for a run. She starts with one city block. She eventually sets her sights on 26.2 miles. Along the way, she confronts body image anxieties, makes friends and turns into an adult.

The plot, by writer-director Paul Downs Colaizzo, might sound like the kind of hokey motivational story that only a proselytizing jogger or a sneaker clerk would dream up. And some might understandably turn their nose up at a movie that increasingly equates exercise with righteousness.

Not all self-renewal comes via a pair of New Balances, but it does the trick for some, and part of the fun of “Brittany Runs a Marathon” is how diametrically opposed Brittany initially is to the idea. She snarls at her fit neighbor (Michaela Watkins), cursing her as “Money Bags Martha” even though her name is Catherine. Faced with the exorbitant fees of a gym membership, she subtly disarms an entire industry by reminding the salesman that, you know, going for a run outside is $0.

Really, the main reason to see “Brittany Runs a Marathon” is Bell. A former “Saturday Night Live” writer, this is her first lead role, and she's stellar in it. Her performance is a deeply layered one, rendering Brittany a very human concoction of sarcasm and bitterness.

Running is only part of Brittany's path, and Colaizzo steers his protagonist through a makeover much more interior than exterior. It takes some likable digressions, too, roping in unconventional supporting players. Utkarsh Ambudkar excels as Jern, Brittany's colleague in a dog-sitting gig. Lil Rel Howery, who gives every film he's a part of a boost, is around occasionally as a brother-in-law. Micah Stock also makes an impression as a jogging pal.

“Brittany Runs a Marathon” starts comically, but it grows increasingly earnest. That's part of the movie's charm but also what leads it a little off track. It doesn't exactly lose its footing — Bell keeps a hold of her character too firmly — but some of Brittany's personality slips away. Losing weight doesn't mean you have to lose your sense of humor, too.

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Starring: Jillian Bell, Utkarsh Ambudkar, Michaela Watkins, Lil Rel Howery, Micah Stock

Directed by: Paul Downs Colaizzo

Other: An Amazon Studios release. Rated PG-13 for language, sexuality and some drug material. 103 minutes