SALT LAKE CITY (CNN) — Dan Hyatt's wife, Shelley, had been worried for a while by the time he hit 337 pounds.

Her husband had a short fuse and a bad attitude because he hated the way he looked and felt. He was on multiple medications for high blood pressure and used a CPAP machine at night to treat his sleep apnea. She tried to dance around the problem, but she was afraid of becoming a widow.

"A couple times I said something like, 'How much life insurance do you have?' " she said. "I was afraid if I said, 'I'm afraid you're going to die,' that would make it come true."

On January 13, 2012, Hyatt crawled to the top of a tower at the plastics plant where he worked, doubled over and struggled to catch his breath. That was when it finally hit him — if he didn't lose weight, he might not live to see his three daughters graduate college, get married or have children of their own.

Heavy all his life from poor eating habits and little exercise, Hyatt had lost weight before only to gain it all back. So this time, he didn't set a weight loss goal.

He gave himself a bigger challenge: Compete in an Ironman triathlon in five years or less. Arguably the world's most grueling endurance race, the Ironman consists of a 2.4-mile swim, a 112-mile bicycle ride and a marathon 26.2-mile run with no break.

"I can't explain how my logic works, but I just decided, if I try to run the Ironman, even if I don't get there, I'm going to lose the weight and I'm going to get fit," said the 44-year-old from Ottawa, Illinois. #hyatt1} Eight months later, Hyatt had completed his first triathlon, run his first marathon and lost nearly 100 pounds. To date, he has lost 135 pounds and 12 inches off his waist by working out 12 to 14 hours per week — swimming, running, cycling and doing yoga — and sticking with a fairly rigid diet.

"I love the fact that I've done what I've done, but it pales in comparison to what I want to do," he said.

Cruel taunts in childhood

Hyatt was overweight as a child. He remembers the other kids calling him "Moby Dick" and yelling, "There she blows." Sports weren't his thing. He liked to swim, but the only time he went jogging was when his concerned parents would make him run around the block to try to help him lose weight. That just brought him more shame and ridicule.

"I'd have people I didn't even know yell names at me — 'Here comes the fat boy running.' "

But decades later, when he laid down the Ironman gauntlet, there was no turning back.

He didn't tell his wife at first. The Monday morning after his January 2012 epiphany, he snuck out of bed and got to the YMCA when it opened at 5:30 a.m. He texted his wife something like: "I just went for a swim."

"I'm like, what? What are you talking about? You don't just go for a swim in the middle of January," she said. As the months went on, she got a clearer picture.

"At first it was just the swimming, and then he comes home and goes, 'I'm going to do a Couch-to-5K.' And then, 'I'm going to take up cycling,' and he signed up for a cycling class. And then he said, 'I'm going to do the Ironman.' "

The workouts escalated slowly. During the first month, Hyatt swam four mornings a week, adding a couple laps every other day until he got to a mile. After a couple weeks, he signed up for a weekly spinning class. When he joined up with some people starting the Couch-to-5K nine-week training program for beginner runners, he was still more than 300 pounds and breathing so heavily that the other joggers would ask if he was OK.

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At work one day, IT consultant Justin Shelton suddenly began to feel ill. A few hours later, he was in the emergency room, and doctors told him he might have a kidney infection.

But he made it to every run, three days a week. He finished his first 5K in 39 minutes and 56 seconds. From there, it was onto a 10K.

Learning to ignore his detractors

Dealing with naysayers was the hardest part. In the beginning, people told him he'd destroy his knees. He was too big to be running, they said. When he hadn't lost all the weight, some people even accused him of lying about swimming a whole mile or running six. He said his wife and two friends were the only ones who believed in him.

A nutritionist helped Hyatt balance his diet around running and cycling.

He sticks to whole grains, lean meats, fruits and vegetables and tries to eat a combination of 55 percent carbohydrates, 30 percent fat and 15 percent protein, per her recommendation.

If the family goes to a Chinese restaurant, he orders steamed chicken and rice with the sauce on the side. He treats himself to a Skinny Cow ice cream sandwich most days, and if he's training for a marathon, he'll eat a few slices of pizza.

In September 2012, he ran a half marathon, and later that month, a triathlon. A couple months later, he did a full marathon.

This past July, he completed a half Ironman, his proudest accomplishment so far. Swimming 1.2 miles in Lake Michigan was "like swimming in a washing machine," and as he swallowed water and felt his chest tighten, he seriously contemplated dropping out of the race. But he kept going.

"When you get to that finish line ... it brought tears to my eyes," he said. "At the same time, it was a stark reality that I'm only at half of where I need to be."

Early to rise: A disciplined regimeNo one doubts him now. He wakes up at 3 a.m. on weekdays to run, swim and strength train before he leaves for work at 6 a.m. On weekends, he's usually up by 6 a.m. to cycle. When the family travels, his first call is to find the nearest YMCA with a pool.

He weighs 202 pounds, about 20 pounds from his goal weight. He has gotten off two blood pressure medications, sleeps better and is a much more pleasant person to be around, his wife reports.

"Nobody would ever say anything to me anymore," he said. "I'm way more fit than the average person these days, and I'm proud of that. And I'm not going to quit."

Hyatt is saving money for the $625 entry fee required to participate in the Louisville Ironman in August. And he won't be stopping there.

After the Ironman, he's got his sights set on ultramarathons, trail running and the Boston Marathon (to qualify, he'd need to shave more than an hour off his current time). He also wants to win a race.

"The human body is capable of so much more than you can fathom if you have the mental fortitude to put past the artificial limits that your brain places on it," he said. "The harder it sounds or the more challenging it is, the more appealing it is to me."

Shelley no longer worries how much life insurance they have, though she does wonder when he'll be home from his long runs or bike rides. On the upside, he cooks dinner for the family nearly every night to make sure the food is prepared healthily.

"There are time when I'm like, 'Oh my gosh, I can't believe you're doing this; I can't believe how insane I am to let you do this,' " she said. But, ultimately, she said, "I'm so happy and so thankful because he's adding years to his life."

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