Frank Ruona, the flinty 71-year-old at the hub of “26.2 to Life,” is not prone to effusiveness. When addressing members of the 1000 Mile Club, Mr. Ruona keeps his remarks to the point: “Less than 1 percent of the population have finished a marathon. So, if you can go out and finish a 26.2-mile marathon you’re … doing pretty good.” Body language tells all, as Mr. Ruona’s hand gestures and general demeanor intimate the strenuousness, if not the impossibility, of the endeavor at hand.

“26.2 to Life” is the debut documentary feature from Christine Yoo, a young woman who has had her hand in a variety of pursuits, including screen-writing, producing, and volunteering at San Quentin State Prison, a facility just north of San Francisco. A 2016 article in GQ by Jesse Katz led Ms. Yoo to the 1000 Mile Club — a runners’ group based at the prison coached by Mr. Ruona and a group of like-minded volunteers. “I am not a prison abolitionist…,” Ms. Yoo writes, “but what I learned from the 1000 Mile Club is that it’s possible to change lives, to make a lasting impact.”

How effective has the running club been in that regard? Ms. Yoo notes that members of the 1000 Mile Club who have had their life sentences commuted and were subsequently released from San Quentin have a zero percent recidivism rate. As for Mr. Ruona, he doesn’t “question any of the inmates about their crimes. What matters to me is what they are doing now and what they are gonna do in the future.” His generosity of spirit, though tightly stated, is nonetheless felt by the men in his charge.

The film begins on the beautiful November day on which the 1000 Mile Club will run a marathon consisting of 105 laps around the prison yard. The track isn’t ideal: The ground is uneven, the course irregular, and runners have to navigate other prisoners, many of whom wander through their pathway. Barely seven minutes into the race, a “yard down” alarm goes out and all incarcerated individuals have to hit the ground. There’s an emergency afoot in the prison. Mr. Ruona is ever laconic: “It’s not a good start.”

Prisoners who sign up for the 1000 Mile Club train all year for the event, building up their physical endurance and taking on a discipline that may be new to them. There are, from all appearances, a good two-dozen men who engage in the marathon, but Ms. Yoo focuses on three individuals: Markelle “The Gazelle” Taylor, Tommy Lee Wickerd, and Rahsaan “New York” Thomas, the latter hailing from the Brownsville section of Brooklyn. Each man committed a serious crime, but each also evinces a gravitas that speaks of innumerable hours spent contemplating the consequences of their actions.

As should be obvious from his nickname, Mr. Taylor is the fleetest of the group. At this point, his story has seen significant media coverage — “The Gazelle” has been the subject of profiles in the New York Times and on CBS — but even those who know how it has played out can’t help but become engrossed in his struggles. 

Mr. Thomas has found renewed purpose as a journalist and podcast host, and it may not be unjust to claim that Mr. Wickerd — with his swastika tattoo, loving wife, and 15 years of being drug- and gang-free — has undergone the greatest transformation. As of this writing, he’s set to be released at age 86.

In terms of cinematic form, Ms. Yoo’s picture is fairly conventional and less encompassing than it could be. A cynic might wonder why other members of the 1000 Mile Club didn’t receive more focus, even taking into account how an individual might want to retain his privacy. Similarly, an emphasis on Mr. Ruona’s military service, his faith, and the unexpected turns of retirement would’ve gone some way in underscoring the ripple effects of generosity. 

Still and all, who doesn’t like a redemption story, particularly when the odds are daunting? “26.2 to Life” is, as Mr. Ruona’s has it, testament to the virtues of being your brother’s keeper.

CORRECTION: Christine Yoo is the name of the filmmaker. The name was incorrect in an earlier version.