Brittany is not doing well, though she thinks she’s fine. An insecure woman in her mid-twenties, she loves to hit the clubs all night and sleep half the day away, then roll in late and hungover to her job, doing not much more than killing time until it’s time to go out and party again. She goes to a doctor to try to get an Adderall prescription and he refuses, instead telling her that she needs to lose weight.

Brittany thinks the doctor is fat-shaming her (“I feel like you’re totally missing the point of those Dove ads!”), but he insists that it’s a serious health problem, and she needs to take off 50 pounds or so, starting right away. She responds, “That’s the weight of a Siberian husky!” But the doctor won’t give her the pills, so she slumps back to her apartment, where her shallow roomate Gretchen is a fellow partier whose life goal is to be nothing more than a social media influencer.

Catherine, a neighbor, is a runner who urges her to come along one day, but the thought terrifies Brittany, particularly since she’s not too fond of Catherine. But since she can’t afford a gym membership, Brittany decides to try running on her own, literally one block at a time. Each day, she goes a little bit further, until she has enough confidence to join Catherine’s running club in a park, where she bonds with Seth, an equally out-of-shape guy who took up jogging after he hyperventilated during the sack race at his son’s school.

Meanwhile, Brittany needs money, so she takes a job as a house-and-pet-sitter for a wealthy couple that’s out of town. That’s where she meets Jern, a guy who’s supposed to only be there at night to watch the dog, but has moved into the townhouse. He’s lost in life, too, with the same cockiness and arrogance she has, so they clash at first, but eventually grow closer.

Throughout, Brittany retains a sharp wit, a defense mechanism that helps her deal with all of these situations and relationships. As she keeps running and dropping pounds, she starts to develop some self-esteem, too, to the point where, yes, she decides that she and Catherine and Seth should enter the New York Marathon.

“Brittany Runs A Marathon” has two big things going for it.

First and foremost is the breakout performance of Jillian Bell. I hadn’t seen her in anything else, so I was pleasantly surprised to be introduced to a comic actress with fantastic timing who also made Brittany vulnerable enough for us to empathize with — but not in a sappy way. Bell — who took up running for real and lost quite a bit of weight while making “BRAM” — is certain to get lots of attention from both indie and mainstream moviemakers, guaranteeing her other lead roles for the next few years. The supporting cast (particularly Michaela Watkins as Catherine) is really good, too.

Second is the writing of playwright-turned-first-time-movie-director Paul Downs Colaizzo, who based the movie on the true story of his real-life best friend Brittany, and created a script that doesn’t always take the plot down the obvious paths. This could easily have been a syrupy Lifetime motivational movie, but it avoids those melodramatic potholes nicely by emphasizing Brittany’s mental changes as well as the physical. Yes, it’s inspirational, but it’s also very funny — one of the best comedies of the year.

I give “Brittany Runs A Marathon” an 8.5 out of 10.