A fun ride, spiked with touching gravity, is not a shabby way to end the movie summer. Thanks to Jillian Bell, a comic force of nature with real dramatic chops, that’s what you get in Brittany Runs a Marathon. The glorious Bell, who stole every scene she graced in 22 Jump Street, plays the title role, an aggressively upbeat twentysomething who gets a medical wake-up call about weight, high blood pressure, and Adderall addiction – her body mass index is in the obese zone. Brittany barely gets by working as an usher at an Off-Broadway theater, and her lack of ambition and direction is eating away at her self-esteem. There’s a downside to being what she calls “the fat sidekick” to her skinny, narcissistic roommate Gretchen (Alice Lee), and it’s not pretty. For Brittany — and anyone who lives in the real world — self-improvement is all baby steps and the same goes for the movie, which refuses to leave Brittany’s demons unexplored. It’s the backsliding that gets to the roots of her self-loathing. In one stunner of a scene, Brittany boozes it up on a visit to the home of her supportive sister and brother-in-law and insults their overweight guest, a woman who is bearing the brunt of all the hate Brittany actually feels for herself It’s running the 26-mile New York City Marathon that represents Brittany’s impossible dream, especially since her attempt to merely jog around the block leaves her out of breath and on the verge of collapse. Still, she has a nearly year to prep and a newfound determination. Luckily, debuting director Paul Downs Colaizzo, an acclaimed playwright (Really Really) who based the film on the experiences of his own best friend (also named Brittany), doesn’t build his sharply witty script for easy sitcom miracles that tie everything up in a neat Hollywood bow. It’s true that Brittany creates a support team in another inexperienced runner, Seth (Micah Stock), and acid-tongued expert Catherine (Michaela Watkins), who puts them both to shame. And she does find love, of sorts, with snarky Jern (Utkarsh Ambudkar, terrific), who shares the house- and dog-sitting job she takes to pay for gym class. You just know their sniping byplay has to lead somewhere; that it doesn’t follow the cliché handbook is a bonus. For Brittany – and anyone who lives in the real world – self-improvement is all baby steps, and the same goes for the movie, which refuses to leave Brittany’s demons unexplored. It’s the backsliding that gets to the roots of her self-loathing. In one stunner of a scene, Brittany boozes it up on a visit to the home of her supportive sister and brother-in-law and insults their overweight guest, a woman who is bearing the brunt of all the hate Brittany actually feels for herself. Bell and Colaizzo do it the hard way, creating a character to root for without glib shortcuts. When you cheer at the end – and you will – the laughs and the tears feel honest and richly earned. Yes, Brittany Runs a Marathon gets a little earnest at times, stopping just short of self-righteousness. But Bell is always there to pull us back. She is pure gold.