During some early scenes, you’d be forgiven for thinking that “Brittany Runs a Marathon” is yet another night-of-drunken-antics comedy — with Fireball shots, misguided sex and ample one-liners.

But the terrific movie sobers up fast.

Brittany Forgler (Jillian Bell) is a combination of Tiffany Haddish in “Girls Trip” and Zach Galifianakis in “The Hangover”: an unrepentant New York party animal quickly approaching her 30s, and getting more careless with each passing night. Her regular routine is that of Ke$ha’s “TiK ToK,” only she does not wake up in the morning feeling like P. Diddy.

One day, while visiting a new doctor in an attempt to get an Adderall prescription, she gets told her body mass index and the MD tells her she needs to lose 45 pounds.

“That’s the weight of a Siberian husky!” she replies.

After tearfully examining her dangerous life choices, and through the prodding of a fitness-freak neighbor, Brittany joins an Astoria running group. Hesitant at first, she eventually makes new friends, who are healthy in more ways than one, and gradually ups her goal to running the New York Marathon.

If that seems like a straightforward, feel-good path, know that the movie has as many turns as the 26.2-mile, five-borough race. Although her appearance and daily life improve, she loses her old friends. Brittany’s relationship with her fun, Instagram-influencer roomie, she realizes, was based on mutually assured destruction, so she says sayonara. And her budding romance surprised me like a Citi Bike going the wrong way.

“Brittany” is a breakout role for Bell, who’s previously been resigned to bit parts in broader comedy films and TV — some quite memorable. (She was Larry David’s assistant on the “Curb Your Enthusiasm” episode “The Bare Midriff.”) But this is subtler stuff. Her touching, egoless performance effortlessly channels a generation that feels stunted and looks in the wrong places to fill the void. She’s joined by a perfectly uptight Michaela Watkins as her neighbor and eventual buddy, and a very funny Utkarsh Ambudkar as an accidental roommate.

The film, from Amazon Studios, is written and directed by Paul Downs Colaizzo, who never goes for the easy joke. Sure, “Brittany” is filled with witty lines. My favorite: “I tried to rescue a dog from a kill shelter — and they said I couldn’t give it the future it deserves.” But there’s no mocking physical humor here, and her rowdy nights out are more sad than silly. “Brittany” is a comedy so relatable for young New Yorkers, you might need to take a walk after watching it.

The tone of “Brittany,” and its emotional impact, reminds me of Amazon’s other heartfelt winner, “The Big Sick,” which netted Kumail Nanjiani and Emily V. Gordon an Oscar nod for original screenplay.

Colaizzo should get one, too.