Marcus Cook is literally half the man he used to be.

And that’s a good thing.

In only a year and a half, Cook has dropped from 489 lbs down to 233—an impressive drop of 256 pounds for the 5'8" man—and whittling his frame from 70.2% body fat to a far more reasonable 33.4%. And while Cook attributes the first 100 lbs to the gastric bypass surgery he had in September 2015, the rest is a product of good diet—and a whole lot of miles logged swimming, biking, and running.

All that hard work paid off for the Houston-based oil and gas executive on Jan. 15, when Cook finally realized his boyhood dream of finishing the Houston Chevron Marathon in his hometown.

“Growing up in Houston, I wanted to be a runner at the age of 8 or 10,” says the 44-year-old, who finished the marathon in 6:12:22. Years later, and more than a hundred pounds lighter, “coming into downtown Houston into the chute was an awe-inspiring moment. It was a dream fulfilled from when I was a young child.”

Even better, Cook’s newfound health gave him a new dream: completing the Ironman Triathlon.

On Sunday, April 2, Cook finished the Galveston Half-Ironman in a respectable 7:39:55. A few weeks later, on Saturday, April 22, he went the full distance, completing the full Texas Ironman triathlon—a 2.4-mile swim, a 112-mile ride, and a full 26.2-mile marathon—in 16:31:59.

The Ironman team documented his journey in a video, and trust us, it's worth watching:

But back in 2015, Ironman triathlons seemed all but impossible. Here’s what inspired Cook to take control of his weight—and how he did it.

Men’s Fitness: How did you get up to 489?

Marcus Cook: I was raised in a household where eating was a social event, and so I ate socially for years—and ate the wrong things—until, one day, I simply became extremely obese. That extreme obesity led to a lack of exercise, which just kept feeding into the obesity.

What sparked your desire to lose weight?

I had been very successful in business my whole adult life. When a close business mentor of mine was diagnosed with cancer, he told me he was dying from an incurable disease, while I was dying because of a choice. And so I took that statement and said, “Enough is enough. I’ve got to do something.”

How much do you attribute your weight loss to the surgery, to good diet, and to exercise?

I attribute the first 100 pounds to surgery and then the rest to exercise and diet. But surgery is just a tool. To change who you are, you have to change everything. You’re no longer “the big guy”—you’re an athlete. No matter what you look like in the mirror, you have to start thinking like an athlete. You can’t think like you’re on a diet or had surgery or a quick fix. It’s a change. It’s like you’re entering the witness protection program for fat people.

How much have you reduced your caloric intake since those days?

I was probably drinking seven Sprites a day and eating whatever I wanted throughout the day. I was probably consuming about 5,000 to 6,500 calories. Now, I’m vegetarian, and only consume clean calories—about 1,500 to 2,000 a day, plant-based.

I’m also working out about two hours a day. I’m probably swimming three miles a week and biking around 150 miles a week and running about 23-24 miles a week.

"He was dying from an incurable disease, while I was dying because of a choice. I said, 'Enough is enough.'"

What was your first workout like?

The team I got around me told me to walk 20 minutes a day. And so that’s what I did. I walked 20 minutes a day for two weeks and I thought I was going to die. I’d never walked or run except to get away from something, or toward the dinner table.

What has been the toughest part of all of this?

The toughest thing is just simply doing it. The simple task of just knowing you have to wake up tomorrow and swim a mile. But it could be something as simple and easy as walking 20 minutes whenever you haven’t walked in 20 years.

What is your end goal for weight?

I have no goal weight. My goal weight is meaningless at this point. Now it’s about what I want to do. My ultimate goal is in two or three years to qualify for the Boston Marathon. If I’m 160, I’m 160. If I’m 175, I’m 175. I will have skin surgery to take off skin. I have about 60-65 pounds of excess skin that needs to be surgically removed.

What impact has having a trainer had on you?

Having a coach is a night-and-day difference from doing it by myself. I’ve tried to do things by myself my whole life with fitness and I’ve failed every time.

How has this journey affected your mental health?

Mentally, it’s opened my eyes up to how much I’ve missed. Losing weight like this brings a new youthful attitude to your life that you can do things you’ve never been able to do for 20, 25 years.

How has this journey affected others—including your family members?

Whenever I was overweight, I thought I was just hurting myself. My son was 80 lbs overweight. Now that I’ve lost my weight, he’s lost 80 lbs himself. He finished his first marathon at 14 years old.

How did it feel to cross the finish line of the half-Ironman?

A year ago I went to the race to see what a triathlon was all about. I’d never been. Whenever I was on the couch, I’d watch some motivational stories out of the Ironman and thought, “Maybe I could do that.” So coming across the finish line, feeling the feeling that I saw so many people having last year, and knowing that three weeks from now I would be crossing the line and hearing “Marcus Cook from Houston, Texas, you are an Ironman!”—it kind of hit home.

What are your goals after the Texas Ironman?

We’ll do Texas one more time and I’ll crush that time by hours [Cook’s half-Ironman time was 7:39:55] and then I’ll do Kona [for the Hawaii World Championships]. I’ve raised $60,000 for Ironman so far. I’m trying to raise $100,000.

What advice would you give to those who are obese and who might have given up?

Just do something new every day. That’s how you change. You change by stopping what you used to do and doing something different. If you do what you always do, you’ll get what you always got.