The streets near Marcus Cook’s home in Houston had become murky rivers, filled with debris and people trying to escape their flooded houses.

As Hurricane Harvey drowned the Texan metropolis in a deluge, Cook glimpsed a man struggling to pull an elderly woman in an inflatable kayak through the rushing, chest-high floodwaters. She lost her balance, flipped the kayak, and submerged, the 15-mph current sweeping her and her possessions into the murk.

Cook did the one thing he could: He jumped in after her, reached her, and finally helped pull her to safety.

Had Hurricane Harvey hit two years ago, though, Cook would have been on the couch, watching news and weather reports. He couldn’t have jumped, much less carried a woman through floodwaters. He couldn’t have helped the 100 other people he and his son Zach helped in the wake of the storm. Two years ago Cook would have been raiding his freezer during the power outage instead of feeding hundreds of victims and volunteers in the weeks after the hurricane.

Two years ago Cook weighed 489lbs; he would have been the one who needed saving.

Back then, though, Cook's rescue came in the form of a friend with an unusual request. Chuck Dalio, his boss in the pipeline supply business, had cancer. He pleaded with Cook to make a change.

“He told me he was dying of an incurable disease, but I was dying because of a choice,” Cook says. So, in September 2015, Cook underwent gastric bypass surgery, a risky decision that nevertheless set him on the right path to losing weight.

Now, thousands of little choices about diet and exercise later, Cook weighs in at 217lbs—fit enough to both finish an Ironman triathlon and help out his neighbors in a hurricane.

The journey has not been easy. His first exercise was a 20-minute gut-wrenching half-mile walk. “I thought I was going to die,” Cook says.

But he didn’t die, and he didn’t quit. He kept walking—then started jogging and biking and swimming. On the one-year anniversary of his surgery, he ran a half-marathon. Four months later, in January 2017, he completed his first marathon. In early April, he swam 1.2 miles, biked 56 miles, and ran 13.1 miles to survive a half-Ironman race. Three weeks after that, the once nearly 500-lb man conquered the ultimate fitness challenge: the Ironman triathlon.

That’s how, in the grueling days of a Houston summer, Cook found himself lending a hand to a flooded city desperate for any help it could get.

“We walked into that same community that we had the boat in, and we started ripping up carpets and tearing out drywall for people,” Cook says. “My daughters and my two sons and my wife. As a family, we’ve never done that.”

He and his family continued to help in the days afterward.

“He always had a heart for people,” his wife, Mandy, says, “but two years ago there’s no way on earth he could have been out there.”

It didn’t end there. Through his social media contacts gained on his weight-loss and Ironman journey, Cook was able to help feed those affected by the disaster.

“I had an [email] from a guy [Paul Ernewein] who follows me on Facebook who can cook up to 1,000 meals at a time,” Cook says. “Because of the Ironman connection, my daughter, my nephew, and I delivered 600 meals in one day. You change your health, and all of a sudden, the people you can help have been changed. The decision to change my health helped change the world, in a better way.”

Cook's transformation has had a ripple effect within his family, too. His 22-year-old daughter, Ciara, has completed two half-marathons. Zach dropped 50lbs and has a half-marathon and four sprint triathlons under his belt, while Zach’s 15-year-old brother Jackson lost 80lbs and has already finished a marathon. Cook’s wife had completed 5Ks and was signed up to run a half-marathon before being sidelined with a broken ankle. Even little 10-year-old Emma, who always nabs a kiss from dad right before his finish lines, has gotten in on the act, competing in two 5Ks and multiple kids’ triathlons.

For Zach, who hopes to follow in his father’s footsteps—literally and figuratively—by finishing the Houston Marathon and the Texas Ironman next year, the physical and mental changes in his father have altered the Cook household in amazing ways.

“It’s been remarkable,” says Zach, who has embraced a plant-based diet like his father. “Going from having to pick him off the couch to watching him cross the Ironman finish line—it’s been crazy watching him transform into this whole new guy. He’s been a lot more involved, and we’ve trained together, run together. It’s been a whole new bonding experience.”

Mandy agrees: “He was always a good husband and a good dad, but he’s more active now. Not that he didn’t want to be at their activities before, but he felt he would embarrass his kids if he were there.”

There’s been a mental change, too: “His self-confidence has gone from nothing to sometimes more than it should be,” Mandy jokes.

That confidence, coupled with an incredible drive, is what has led him to succeed where others have failed, according to his coach, John Muse, who has worked with him for most of the last two years.

“When he sets his mind to something, he is going to get there in whatever way he can,” says Muse, who has taken him on 14,000-foot mountain hikes when Cook visited him in Boulder, CO. “It’s very, very self-driven. There’s a moment of truth, there’s a realization that if you don’t make a change, then you might die. That is the catalyst for yourself moving forward. And some people can make the change, moving slowly and progressively forward, and others just have a hard time making that change.”

Cook plans to keep moving forward, hoping to shave hours off his marathon and Ironman times as he replicates his Houston Marathon and Texas Ironman races again next year. His ultimate goal is raising $100,000 for the Ironman Foundation, gaining a special entry into the Ironman World Championships in Hawaii.

“Making that decision to change—you think it’s all about me, but then you realize it affects everything," he says. "As men we segment our lives in pockets. You have your family pocket, you have your work pocket, you have your hobby pocket. But if any one of those little areas is unhappy or needs change, they affect one another. Fixing my health made my family better. Fixing my health made my relationship with my wife better.”