Eleven years ago, Martinus Evans’ doctor issued him a dire warning.

“You’re fat. You can either lose weight or die,” he told Evans, then 25 and 360 pounds.

Evans, a former college football player, made a bold promise to himself and his doctor: He would run a marathon.

At first, the Brooklynite could only run 15 seconds at a time on the treadmill, but he had what he calls a “delusional self-belief.”

That, and an innate need to prove people wrong — a now even-more-worried doctor among them — motivated Evans to keep going.

Now 36 and weighing 300 pounds, Evans has run eight marathons and over 100 other races.

“Running has opened me up to think … what are other things that I might want to do that I didn’t think I could do … because of my body size and my body shape?” he told The Post.

Evans’ journey has also led him to become a certified running coach, leader of the Slow AF Run Club community (a virtual community for slower-paced runners counting more than 10,000 members worldwide), and the author of “Slow AF Run Club: The Ultimate Guide for Anyone Who Wants to Run,” published by Random House — all, he says, to encourage people to start running in the body they have today.

The humorous title came from an experience Evans had while running the New York City Marathon in 2018.

While crossing the Queensboro Bridge into Manhattan, he took off his headphones after noticing a man yelling and pointing at him as Evans powered through his 16th mile.

“You slow as f-ck! Go home,” the random onlooker shouted.

“For me, I was like, ‘Wait a minute. I’m running the race. You’re on the sideline. Why are you telling me to go home? No, you go home, because I paid to be here and I deserve to be here.'”

The next year, Evans ran the race with “Slow AF” emblazoned on this T-shirt — as a metaphorical “middle finger” to his hater.

These days, Evans casually runs around 3 to 4 miles at a 14-15 minute per mile pace, despite his weight.

And he’s not in the least bit bothered by his critics, who have told him he’s normalizing obesity.

“I’m promoting what the normal person with a normal body actually looks like, in this day and age.” he said.

Evans, who earned a master’s degree in public health research from the University of Connecticut, noted that regular exercise has provided him with “great” blood pressure, cholesterol, and bone density levels, without the dramatic weight loss.

“When people think about health, the first thing people note is the outward appearance and think ‘Oh, that person’s not healthy, because he’s fat,'” Evans said.

“Everybody can be a runner. It may look different from what they see on TV or from professional runners, but everybody can be a runner,” Evans said.

The running coach, who focuses on all aspects of a client’s health, including nutrition and other forms of exercise, advises first-time runners to begin training in intervals as he did: run for 15 seconds, then walk for a minute and slowly continue running faster and/or longer as possible.

He also encourages them to signup for a road race to “really experience the joys that come along with running.”

Evans pushes running as “a gateway for larger movements and other exercises.”

He noted that it’s not just about the action of running but learning to make healthier life habits and tracking health markers rather than looking for physical changes.

With several marathons under his belt, Evans is now considering training for a triathlon.

“People see themselves in me and say, ‘Dang, you know, I really didn’t think I could do it but seeing you maybe I might.’ So that’s what I run for today.”