In "Brittany Runs a Marathon," Jillian Bell plays the title character, a young woman approaching 30 whose life has stalled into a deadening cycle of partying, meaningless sex, shallow friendships and an off-off-Broadway theater gig that barely keeps her afloat.


As an archetype, Brittany resembles the characters Amy Schumer has played in such makeover comedies as "Trainwreck" and "I Feel Pretty": the funny, resilient but wounded girl whose self-deprecation masks deeper self-loathing, and whose self-sabotage veers precariously toward self-harm.


The title of "Brittany Runs a Marathon" is nothing if not literal: Here, the means of the heroine's salvation is her discovery of running, but the twist is that even that healthy pastime — and the positive changes it engenders — invites new ways for Brittany to indulge her deepest weaknesses.


Written and directed by newcomer Paul Downs Colaizzo, "Brittany Runs a Marathon" is an engaging, modestly amusing, sometimes laugh-out-loud hilarious comedy of manners in which the usual millennial excesses are skewered, from the invidious hellhole of social media to the mendacities of online dating. Photographed in a loose, up-close style, the movie never strays far from the usual advances, reversals and resolutions of similar plots — a familiarity that is either comforting or cliched, depending on the viewer's frame of mind. To his credit, Colaizzo has enlisted a fine ensemble of actors, especially Michaela Watkins, Utkarsh Ambudkar and a quietly revelatory Micah Stock as Brittany's newfound posse.


The best part of "Brittany Runs a Marathon" is that it provides a showcase for Bell, who co-starred with Watkins earlier this summer in the terrific indie "Sword of Trust," and who, before these two movies, has usually been relegated to scene-stealing but all-too-brief supporting roles. Right off the bat, she gives Brittany a quick-witted, improvisatory edge, which eventually is shown to cover up for deep-seated social anxiety. Bell plays all the layers with admirable skill, managing to be tartly funny, abrasively off-putting and wrenchingly vulnerable within the space of just a few moments.


She also pulls off the physical transformation that forms the somewhat contradictory climax of "Brittany Runs a Marathon." This is a movie that is adamantly body-positive ("You totally missed the point of those Dove ads," Brittany complains to a doctor who tells her she's overweight), but it also revels in the fit, lip-glossed, romantically fulfilled butterfly who emerges from her cocoon of red wine, Aderall and shame.


Not content to leave it there, Colaizzo pre-empts his foreordained happy ending just long enough to question how Brittany — and the audience — would precisely define that term. "Brittany Runs a Marathon" is perfunctory, idealized, sometimes awkwardly composed, almost always predictable. But it stays the course, with admirable grit and more than a few entertaining grins.