Last summer, Chris Borland ran 26.2 miles through San Francisco with two other marathon novices and the weary-legged trio made a pledge after finishing: never again.

“We all almost simultaneously said, ‘That’s the last one we’re ever going to do,’” Borland recalled. He then continued: “And here I am running a half (marathon).”

Yes, Borland, who jokes he’s still seeking a runner’s high, will be running 13.1 miles later this month. Why? It’s not to satisfy competitive urges he abandoned when, at 24, he retired from the NFL after one stellar season with the 49ers because of future brain-trauma concerns.

Rather, the former inside linebacker is running to help a generation of ex-NFL players, those who didn’t have seven-figure salaries or know about the brain disease chronic traumatic encephalopathy, or CTE.

Borland will run on a relay team in the Ironman 70.3 Ohio Triathlon in Delaware, Ohio, next Sunday to raise awareness and money for the Gridiron Greats Assistance Fund. The organization, which counts Mike Ditka as president, provides money to help retired NFL players with expenses for injuries suffered during their career.

“These are guys who played in a generation where they didn’t make a lot of money, they had offseason jobs to make ends meet, and they weren’t aware of a lot of the neurological risks,” Borland said.

Borland, 25, did extensive research before he retired 17 months ago, and his findings inspired that decision and many of the choices he has made since.

On Thursday, he will attend the screening of “Requiem for a Running Back,” a documentary detailing the life, career and post-NFL mental-health struggles of late Packers running back Lew Carpenter, who was diagnosed with CTE.

Borland was interviewed for the film and helped the director, Carpenter’s daughter, Rebecca, connect with others. He attended the premiere at the Detroit Film Festival in February and was part of a panel that included Dr. Julian Bailes, a neurosurgeon who was among the first to diagnose CTE in NFL players.

The screening on Thursday will be held at the Carter Center in Atlanta, where Borland recently finished a 10-week unpaid internship. The nonprofit public policy center was founded by Jimmy and Rosalyn Carter. The center’s far-reaching work doesn’t include brain trauma, but Borland believes its efforts to destigmatize mental illness pertain to ex-NFL players.

At 225 pounds, Borland is about 25 pounds lighter than he was as a rookie with the 49ers, but he remains a large NFL figure because of his groundbreaking retirement and advocacy in its aftermath.

In June, USA Today placed him among the league’s 100 most important people (Borland was No. 25, 26 spots ahead of 49ers head coach Chip Kelly). There remains a belief Borland’s decision will influence current NFL players to follow his lead and will trickle down to lower levels, severely diminishing football’s popularity.

Borland, who remains close friends with many of his former 49ers teammates, understands many view him as an antifootball crusader. In fact, he is commonly referred to as “the most dangerous man in football,” which was part of an ESPN headline that made him roll his eyes.

However, less than two years ago, Borland was routinely described as a football junkie, an undersized, too-slow linebacker who overcame his limitations with passion and preparation.

He gets the game’s allure and misses the competition and camaraderie. However, he also possesses a passion for helping others who need assistance. At Wisconsin, he collected 126 community service hours in a seven-month stretch, a total that easily led all the school’s student-athletes.

His current aim isn’t to rail against the NFL, but help those suffering after playing in it. And it’s not always clear the league shares the same mission. In May, the New York Times reported the NFL tried to improperly influence government-overseen concussion research the league funded.

“I don’t even attempt to reconcile what people project onto me,” Borland said. “…I try not to be anti-anything, really. I think it’s important to define yourself in a positive place. I don’t go out of my way to be critical of the NFL, but there are some things they have done where the only accurate objective way to address it is ‘Wow, that’s wrong.’ So I have no hesitation about doing that.”

Borland believes there is no way to eliminate football’s inherent danger, but isn’t advocating for its demise. He understands the confusion.

“There’s so much nuance to it,” Borland said. “You really have to peel back the layers before addressing it appropriately. Just to blanketly say I’m against football is inaccurate. But that’s OK.”

Looking ahead, Borland expects to remain connected to the world of traumatic brain injury and neurological research, but it won’t occupy most of his time in his yet-to-be determined, post-football career.

Shortly after running in next week’s triathlon, Borland will start a one-year interdisciplinary master’s program at Oregon State, which has an emphasis on sociology, political science and psychosocial kinesiology. He will also work to start a nonprofit in his hometown of Dayton, Ohio, that will assist high school athletes from diverse backgrounds. He will apply to doctorate programs in December, but doesn’t have a dream job in mind as he steps back into school.

The last time he was in college, of course, he was a star football player with an NFL future.

Three years later, his body has shrunk, but his voice has grown. What will he do with his education and influence? He’s unsure, but allows that assisting others could be an integral part.

“I do know it will be part of my life; I don’t know if it will be part of my work,” Borland said. “There are a lot of different ways to help people and lot of different jobs where you can give back. That will likely be the case, but I don’t know for certain.”

Eric Branch is a San Francisco Chronicle staff writer. Email: ebranch@sfchronicle.com. Twitter: Eric_Branch

Fundraiser

Chris Borland is particpating in the Ironman 70.3 Ohio Triathlon in Delaware, Ohio, next Sunday to raise money for the Gridiron Greats Assistance Fund, which provides money to help retired NFL players with expenses for injuries suffered during their careers. Borland’s fundraising page is at www.crowdrise.com/otherraces2016/fundraiser/chrisborland.