When she visits her doctor looking for help with her difficulty in having focus, he suggests that due to her elevated resting heart rate and high blood pressure she should lose fifty-some pounds. Despite her natural inclination to avoid strenuous activity, and not liking her upstairs neighbor, Catherine (MICHAELA WATKINS), who's always going for a run, Brittany decides jogging might work for her. In doing so, she meets another person struggling with his health, Seth (MICAH STOCK), and they eventually join Catherine's running group.

After they make a pact to run in the New York City Marathon, Brittany feels comfortable enough confiding in them that she sort of likes a man she sort of works with, Jern (UTKARSH AMBUDKAR), who has the dog-sitting night shift for a well-to-couple that has hired Brittany for the daytime role. He's lazy and aimless but supportive of her, and their work friendship turns into a romance.

But even with Brittany losing weight while getting in shape, she can't shed the negative "fat girl" psyche that's ingrained in her soul. All of which means her constant need to keep others at arm's length might just ruin her progress, new friendships and relationship with Jern.

And it's far more common for male performers to engage in such a body reshaping endeavor than female ones, what with Hollywood and the public's obsession with women's bodies if they don't comply with societal "rules" about what's considered normal or acceptable.

What's rarer, though, is having a performer of either gender purposefully change their body weight in conjunction with their character doing the same over the course of the film. Beyond the "what if it doesn't work" concerns, there's the simple fact that most movies shoot their scenes out of order, meaning continuity becomes an important and, if done incorrectly, quite noticeable issue.

Apparently undeterred by any of those matters, Jillian Bell set out to do just that in losing forty pounds while shooting and appearing as the title character in "Brittany Runs a Marathon." Funny and insightful -- and definitely R-rated -- the film centers around Bell's character who's in her late twenties, doesn't seem to have her act together, and weighs in at nearly 200 pounds on her 5'- 6'' frame.

When she seeks out professional help to deal with her recurring inability to focus, her doc suggests she lose fifty-some pounds, something she points out is the equivalent of shedding a Siberian Husky from her body (which sets the tone for the sort of self-deprecating humor that's going to fill the 100-some minute offering).

Nonetheless, and with her doctor not taking her not-so-subtle bait about prescribing her some Adderall to help her, she laces up her running shoes and, after some last-minute doubt, takes her first steps that will eventually segue into her desire to run in the world-famous New York City marathon.

Along the way she meets another terribly out of shape, would-be runner (Micah Stock) and ends up befriending her neighbor (Michaela Watkins) who she initially secretly despised but now joins in the latter's running club. At the same time, she must contend with her party-hopping roommate (Alice Lee) who condescendingly doubts she's a real runner, as well as a working relationship turned friendship turned romance with her co-worker of sorts (Utkarsh Ambudkar).

But her societally-infused and self-inflicted psyche of "once a fat girl, always a fat girl" threatens to undermine all of those relationships as she trains for the big event. Like most such films, the comedy is lathered on thick early on to engage us, and is slowly but surely replaced with such heavier thematic material.

Bell, who many won't recognize despite appearing in a number of TV shows such as "Workaholics" and "Eastbound & Down," gives off an agreeable, early Amy Schumer like vibe (before that comedian-actress got overexposed and ran her act into the ground), but not to the point of irritation or feeling like a cheap imitation. The chemistry between her and Ambudkar's lazy but good guy character is appealing, and the supporting work by Watkins and Stock nicely adds to the mix.

Not perfect but entertaining, quite funny at times, and thought-provoking when it comes to women, their weight, and society's reaction to that, "Brittany Runs a Marathon" goes the distance, seemingly without breaking a sweat or stumbling over Bell's successful attempt to mirror her character's training and associated weight loss. The film rates as a 6 out of 10.