Marathons have certain prescribed routes, and so do movies like playwright Paul Downs Colaizzo’s debut, a likable but largely predictable dramedy about a woman who changes her life by staking out a challenging goal and working to achieve it despite pitfalls along the way. Based on the experience of Brittany O’Neill, a friend of Colaizzo’s, “Brittany Runs a Marathon” is a decent enough crowd-pleaser, but hardly a groundbreaking one.

Brittany Forgler (Jillian Bell) is introduced as an overweight twenty-something barely making ends meet working as an usher at an off-Broadway theatre and partying all night with her roommate Gretchen (Alice Lee), who is trying for social-media stardom with her boyfriend. Brittany acts happy and fancy-free, but insecurities fester beneath the surface, and when she visits a doctor to get some pills to keep her going, his advice that she undertake to lose some pounds and get her blood pressure under control comes as a wakeup call.

Brittany decides to begin exercising, but gym rates are beyond her, so she decides to take up jogging instead. That throws her together with a neighbor, thin, apparently self-confident Catherine (Michaela Watkins), whom she’s previously dismissed as an uptight bore but who now proves surprisingly sympathetic even as self-absorbed Gretchen doesn’t. Catherine is a member of a running club, and eventually Brittany joins it too, and they become a duo—or more accurately a trio, since another runner, gay Seth (Micah Stock), befriends them as a partner in pain. Eventually the three will work toward running in real races, finally agreeing to attempt what seems a total impossibility—tackling the New York City marathon, a twenty-six mile endurance contest.

Romance also enters the picture as Brittany takers a new job house-sitting for people who don’t want to leave their pets in kennels while they’re away. Her first gig brings her into contact with her employers’ “night guy,” Jern (Utkarsh Ambudkar), a likably irresponsible slacker who’s turned the place into his permanent residence in their absence and suggests she do the same. As implausible as it seems at first, the two hit it off and get close over time.

One of the strengths of Colaizzo’s script is that Brittany isn’t portrayed as all sugar and spice. Even in the early going, she can have a sharp tongue and a readiness to use it. But her aptitude for self-loathing, and a predilection to turn it against others, comes to the fore when her constant training leads to an injury, and the return of her employers causes her loss of a place to stay. She moves in with her supportive sister (Kate Arrington) and brother-in-law (Lil Rel Howery), but her unhappiness is obvious, and it bubbles over in a cruel drunken tirade she directs against one of their guests, a woman heavier than she ever was, who nonetheless seems content with the way she looks.

It goes without saying that Brittany recovers from her funk, issues the appropriate apologies, resumes her training after her injury has healed, and eventually runs the marathon, a year later than she’d originally hoped. The toughness of the event is made palpable, but her friends and family are on hand to cheer her one. The only thing missing from the scene is Rob Schneider yelling, as he used to do in all those early Adam Sandler movies about underdogs triumphing in the end, “You can do it!”

Actually, that would have proven a clever way of pulling the movie back from its last-act earnestness to the quirky appeal of the earlier reels. As it is, we have to be satisfied with a “Marathon” that starts out as an intriguing sprint but loses steam in the later laps.

Even there, however, Bell delivers a strong performance, capturing both the comedic and dramatic aspects of the character—and going through the actual weight-loss process that O’Neill did (though perhaps with some help from the makeup artists). Her gradual slimming down is more impressive than a lot of the computer-generated effects one sees in big Hollywood blockbusters.

Bell’s efforts are seconded by the supporting cast. Watkins is especially convincing, especially when Catherine’s self-doubts come into play (she’s going through a messy divorce and has custody issues to sort through); and while Ambudker, Howery, Stock and Lee tend to italicize things and come on a mite too strong, that could be the result of the uncertain control of a first time-director. Technically, the picture will win no awards, but it’s entirely adequate, with Casey Brooks’s editing maintaining a nice pace.

“Brittany Runs a Marathon” might not take the prize, but it crosses the finish line winded but game.