Mary Cain was born to run.

At 12, she joined her school’s varsity track team — and was faster than the seniors. Soon, she was breaking records and winning championships. 

In 2013, at just 16, she joined Nike’s Oregon Project, run by the famed coach Alberto Salazar. Based at the shoemaker’s Beaverton, Ore., campus, the elite running club offered state-of-the-art training facilities.

As Oregon Project runner Kara Goucher wrote in her 2023 memoir, “Everything we’d need to win was at our fingertips — equipment, massages, medical care, coaching — and if it wasn’t, we could ask for it.”

Even having worked with countless phenomenal runners, Salazar told Cain she had potential to transform the sport. Not long after joining the club, she became the youngest track-and-fielder to make the US World Championship team.

She seemed destined for the Olympics. Then she flamed out. 

Cain failed to qualify for the 2016 Olympics. She left Nike’s elite team and disappeared.

In 2019, Cain made headlines again when she publicly spoke out against the Oregon Project, saying that Salazar and other coaches pressured her to lose weight and train through debilitating injuries. In 2021, she filed a $20 million lawsuit against Salazar and Nike. 

By then, he had already been banned from coaching for four years over doping offenses involving athletes he trained, and SafeSport — the group that monitors abuse and misconduct in the US Olympic and Paralympic communities — had banned him for life for sexual misconduct against Goucher.

Now 29, Cain has written a memoir about her experience. “This Is Not About Running” (Mariner Books), out April 28, delves into the incredible highs and spirit-crushing lows of her time as an athlete as well as the toxic, abusive world of youth sports. 

“I’ve never been drawn to being in the public eye,” Cain told The Post, speaking from her room at Stanford, where she’s now completing her second year of medical school. “But I just felt like this was a story that could actually maybe help people if I told it.”

As Cain puts it in the fiery introduction to her book, her memoir is not about running, but “about how sports normalizes the abuse of young athletes.”

“Whether it’s the sports executives who monetize the bodies of others, the coaches who are given carte blanche control of young people, the teammates who mistreat one another all for a spot on a team, the media that denigrates athletes for article clicks, or the fans who develop unhealthy parasocial relationships with strangers — sports normalizes cruelty.”

The problem, she told The Post, extends far beyond running. 

“I wanted people to feel like you don’t have to be a runner to pick up this book and find value in it,” she said. “We know this stuff happens in, really, any field in which someone enters with a dream. That’s how people end up having these situations where they’re taken advantage of.”

Cain didn’t necessarily choose running. It chose her.

“I was always a very physical kid: I loved running around, I loved playing tag,” she recalled.

Growing up in Westchester County, NY, Cain and her three sisters were first encouraged by their mom to take up swimming. She was in her school’s swim club before someone noted her incredible speed on the track and suggested she try out for the track team.

She was a natural. “What I loved about running is you’re so locked in the moment,” Cain said. “You can’t be multitasking in the way that so much of life pulls you in these different directions. I think I found that very freeing as a kid.”

It was so freeing that she continued to do it, despite problems with the culture at Bronxville High School.

The coach there, a man in his 60s, would pass over Cain for opportunities, she writes — then ask the older girls about boys and parties, which made Cain uncomfortable. Her teammates taunted and bullied her, excluding her from post-race dinners and events. Worse, their parents hurled their abusive invective at Cain.

By tenth grade, it was so bad that, Cain said, her parents considered enrolling her in a different school.

“It got to the point where there were literal safety concerns because [team] parents were just yelling at me,” she said. “There was one incident where I thought one woman would hit me.”

Cain recalled that another mother was standing by, frozen in shock, as it happened. “She looked actually probably even more frightened than I was … but she didn’t say anything,” the athlete said. “She didn’t say stop. She didn’t say, ‘That was wrong.’ She didn’t find me afterwards. Like, I literally ran away hysterically crying, and there was no acknowledgement that that had happened.”

She said that several of these parents were runners themselves and were “vicariously living through their children.”

Still, she didn’t back away. “I had this Disney Channel mindset, like, ‘They are clearly the bad guys. Why should I have to leave?’ ” Cain said. “I think there was kind of that mix of confidence, faith that things could get better, but also righteousness.”

Then, in October 2012 — the same year she broke the American high school girl’s outdoor record in the 1500 meter by more than three seconds — she received a call from Salazar just as she was starting 11th grade.

Salazar himself had won the New York Marathon three times and the Boston Marathon once in the 1980s. Two of the runners he coached on the Oregon Project had just received a gold and silver medal at the 2012 London Olympics. Now, he was asking her to move to Portland to live and train with the club. 

Cain did not want to leave her parents but agreed to be coached by him remotely, with in-person sessions when she could get to Beaverton.

At first, things were great. Cain was winning race after race and regularly beating her previous personal records. Salazar frequently praised her.

When the team’s sports psychologist made her feel uneasy, she tried not to think too hard about it. Ditto when Salazar tried talking with her about his marriage problems and sex life — and when, while staying at his house in Portland, she once woke up to find him in her room.

“I think what’s so difficult and so normal in abuse is that it doesn’t actually start from nowhere,” Cain said. “It’s taken me a long time to maybe feel comfortable saying this, but Alberto sent me the gloves of a woman he sexually assaulted, probably the day he took me on. And I remember opening up that package and thinking, ‘Oh my god, this is so cool. These are Kara Goucher’s gloves, like, this is the greatest gift ever.’

“Now it kind of does sour truly everything, because there’s so many moments like that.”

Things took an obvious turn, Cain said, after she suffered a stress fracture in her senior year of high school. She took a break from running to heal and, when she arrived back in Portland, Salazar obsessively began zeroing in on her weight. 

After high school, Cain enrolled at the University of Portland so she could continue to compete as a pro athlete for Nike — but that meant there was no escape from Salazar’s constant insults about her body, from saying she looked too heavy to run quickly to comparing her to other women.

“It was always going to be problematic to try to have me be underweight, but I think for [him] to also be like, ‘Your butt doesn’t look like this other woman,’ or ‘Your boobs don’t look like this other woman,’ that’s when all of a sudden you’re judging how I look beyond just the number on the scale,” she said.

Salazar, she writes, demanded that she take diuretics to lose weight and restricted what she ate. He talked with reporters about her weight.

Cain started cutting herself and having suicidal thoughts. When she admitted this to Salazar and his sports psychologist, they told her to go to bed.

No longer breaking records and winning awards, she was afraid that if she told her parents, they would make her come back to New York.

Cain finally left the Nike Oregon Project in 2016, but so much damage had already been done.

At 20 years old, she was underweight, had low bone density, and had lost her period. In 2019, she was diagnosed with REDs, or Relative Energy Deficiency in Sport. Two years later, she was found to have functional popliteal artery entrapment syndrome (fPAES) and lost feeling in her foot due to an artery in the back of the leg closing up. 

“I had this for seven years,” Cain said. 

She settled a reported $20 million lawsuit against Salazar and Nike in 2023, after alleging the coach had emotionally and physically abused her, and that the company did not protect her.

Cain had surgery for the condition at Stanford, and the experience inspired her to apply to medical school there and work with the doctor who performed her surgery. “I’ve actually done research with him over the summer that will be published in the next month or two on this,” she said excitedly. 

In addition to medical school, Cain is the founder of a nonprofit called Atalanta NYC, which employs female pro runners to serve as mentors to girls in underserved parts of the city. She’s also on the board of Athlete Survivors’ Assist, an organization working to end abuse in sports.

And she’s even taken up running again.

“This past year has been this really incredible journey where I think I’m at like, four or five months at this point of just like, hitting the same exact amount of mileage week in and week out, and feeling so good and so resilient,” she said. “That’s something I don’t take for granted, because it hasn’t always been like this.”