The life-changing moment that turned 'couch potato' dad into vegan ultra-endurance athlete Rich Roll spoke with ABC's Dan Harris for his "10% Happier" podcast.

 -- By the time he was 39, Rich Roll seems to have everything.

He was a successful entertainment attorney who had worked his way up to partnership and was living in Southern California, where he and his wife had built a family together with four children.

But shortly before his 40th birthday, Roll was climbing a flight of stairs on his way up to bed, when suddenly he became winded, sweaty and he felt tightness in his chest.

“[It] really felt like I was on the precipice of having a heart attack,” Roll told ABC News' Dan Harris during an interview for his “10% Happier” podcast. “[It was] like a very crystallized moment in time where I realized … I have an opportunity to really change my life.”

“That was the beginning of trying to course-correct how I was living,” he added.

Subscribe and listen to the "10% Happier" podcast on Apple Podcasts, Google Play Music, TuneIn, on ABC Radio podcasts and under the "Listen" tab on the ABC News app.

Roll had been a world-ranked competitive swimmer at Stanford University, but his career was marred by a struggle with drugs and alcohol until he ultimately went to rehab at age 31.

“I had the world by the tail at one point and I squandered a lot of those opportunities as a result of my issues with substance,” he said. “When I got sober, I became very intent upon repairing all the wreckage that I created as a result of my drinking and using.”

Roll said he threw himself into work, but stopped taking care of himself. He was 50 pounds overweight when he had his epiphany and immediately committed himself to a healthier lifestyle.

“I was trying everything, I did a juice cleanse,” he said. “I tried a bunch of different diets and really stumbled into eating plant-based as a last resort after trying everything else and not finding success.”

When he became dedicated to a vegan, or plant-based, diet, Roll said he felt “unbelievably better” in less than two weeks.

“I felt I had more energy than I had since I was a teenager,” he said. “I realized I had kind of backed into something that perhaps could be quite profound ... and I had an impulse and a desire to get fit once again, to exercise, which I hadn’t felt in years.”

Roll said he went from being a “couch potato” to someone who was “really excited about being outdoors and going out on the trail at dawn.” That led him to the world of ultra-endurance sports. In 2008, after only six months of training, Roll became the first vegan to complete the Ultraman World Championship -- a three-day, 320-mile double Ironman-distance triathlon in Hawaii -- finishing in 11th place overall and with the second-fastest swim time.

Roll, now 51, has gone on to compete in ultra-endurance race events all over the world. He eventually quit practicing law, and today he said his life consists of training and competing, traveling for public speaking, hosting his podcast, “The Rich Roll Podcast,” and a regular meditation practice -- something he was introduced to while in rehab.

“I believe in it strongly,” he said. “I think part of my evolution was for many years as an ultra-endurance athlete, like I spent a lot of time in solitude training, like lots of hours on the bike, really long runs, and there’s certainly an active meditation component to that.”

“But when I actually committed to doing a formal meditation practice, I realized the difference,” Roll continued. “It just became very clear to me that this was a blind spot I really needed to incorporate into my life.”

Roll has a memoir called “Finding Ultra” and has published a cookbook with his wife called “The Powerplant Way,” which is focused on plant-based eating.

Subscribe and listen to the "10% Happier" podcast on Apple Podcasts, Google Play Music, TuneIn, on ABC Radio podcasts and under the "Listen" tab on the ABC News app.