We know exactly where a movie like “Brittany Runs a Marathon” is going. The uplifting ending that a movie like this is aiming for is as obvious as, well, the finish line at the New York City marathon.

But writer-director Paul Downs Colaizzo, aided by a complex and funny lead performance by Jillian Bell, lets his debut film take a few unexpected detours and side routes along the way. It makes for a movie that’s maybe sadder and more honest than expected, and one that’s wise to both the possibilities and limits of personal transformation.

Bell plays Brittany, a hard-partying 28-year-old in New York City who aspires to be the funniest, loudest person in any room, at any time. Making people laugh is a defense mechanism, a way to shift attention away from herself and back onto the other person. It’s only when she’s staring into the mirror after a long night at the clubs, or scrolling through the Instagram stories detailing her friends’ more fulfilling lives, that her more vulnerable side comes out.

Her doctor says she’s become dangerously overweight, and ought to lose at least 50 pounds. Brittany is appropriately horrified, but after a couple of false starts is able to drag herself out the door and run a block. Then two blocks. Before she knows it, she’s running two miles with a neighborhood running club and making new friends (Michaela Watkins and Micah Stock) who don’t need her to be the center of attention all the time.

As Brittany begins training for the New York City Marathon, learning self-discipline and personal responsibility for the first time in her life, the physical transformation we see on screen (Bell lost 40 pounds over the course of making the movie) is striking. But it’s really Brittany’s emotional transformation that Colaizzo is interested in. Changing your mindset and learning to accept yourself as you are isn’t as simple as cutting back on carbs and doing burpees.

It’s here that Bell, a former “Saturday Night Live” writer who has had scene-stealing supporting roles in “22 Jump Street” and Comedy Central’s “Workaholics,” really shines. Bell can handle the self-deprecating one-liners in Colaizzo’s screenplay with ease, particularly in the first half of the film. But beneath Brittany’s outgoing sense of humor is a layer of bitterness and self-loathing, which she sometimes redirects viciously against people in her life who try to support her. (“I don’t want your pity,” she snarls more than once.) Bell reveals the sometimes unpleasant, self-defeating aspects of Brittany’s character, and her struggle to overcome them, without ever losing our empathy.

It seems silly to worry about spoiling the ending to a movie called “Brittany Runs a Marathon.” But whether or not she makes it 26.2 miles is a bit of a distraction anyway. Colaizzo’s engaging film is really about having the courage and self-confidence to run a much bigger race. He presents a flawed but likable main character who’d we root for just for getting up and hitting the streets every day.

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Rob Thomas is the features editor and social media editor for the Capital Times, as well as its film critic. He joined the Cap Times in 1999 and has written about movies, music, food and books.