(CNN) -- All that stood between Tim McCarty and the thrill-promising roller coaster was a seat belt. If he could just buckle it, he and his children would zoom to 53 miles an hour in the first three seconds.
There was just one problem: McCarty was too big. He had to step out and go tell his wife he weighed too much, while his kids blasted off on the ride without him.
"From there, we kind of made a saying: 'The walk of shame,' " he said. "I realized at that time I was just too big."
Incidents such as this prompted McCarty, 42, of Dallas, to finally take action against his weight. Since September 2012, he has dropped 125 pounds.
"It's not easy. You didn't get big overnight, so it's going to take a little time," McCarty tells others who want to follow his lead. "It's going to take some work. You just got to decide for yourself that's what you want."
At 5-foot-3, McCarty, who works in maintenance at an office building, describes himself as a "short, round person." But it wasn't always that way. He was initially a skinny child, but he gained a lot of weight between the second and third grades. That summer, he stayed home and watched TV and ate.
Other children made fun of him, calling him "sheep dog" and "fat boy."
McCarty knew the health risks of being overweight. He has always been diligent about annual physicals because his father's siblings died of cardiovascular problems, and his father has had a heart attack. Yet McCarty remained healthy in adulthood by other measures. His blood pressure and cholesterol were fine, so he never consistently weighed himself. He thinks he reached 300 pounds in his 40s.
"I thought I didn't need to lose weight because I'm healthy," he said.
At one annual physical, McCarty's doctor told him he still needed to lose weight and that he was "very lucky " to have normal vital signs, but they wouldn't necessarily stay that way.
The doctor said, McCarty remembers, "You're going to be in a world of hurt."
But the message didn't make a dent in McCarty's unhealthy habits until last August, when he decided to race his youngest daughter for one mile. He thought he would finish in about 10 minutes. He estimates he weighed between 310 and 320 pounds at that point.
"It was horrible," he said. "It took me 22 minutes to do a mile." And because of poor route planning, McCarty couldn't make it back; he had to call his wife to pick him up.
The next day, he told his wife: No more sodas. He would start walking. It was time for a change.
A big help to McCarty's weight loss effort was the arrival of a new gym, Downsize Fitness, behind his workplace. The facility is specifically geared toward overweight and obese people, and their equipment and workouts are tailored for larger bodies.
McCarty didn't realize before going to this gym that he could modify challenging exercises, such as push-ups and jumping jacks, to accommodate his fitness level.
He also liked that everyone there had the same goal: weight loss. There were no perfectly toned bodybuilders in sight.
The gym made exercise personal. He'd get calls from employees asking what time he'd be showing up.
"I wasn't just a number there," he said. "They knew what level I was at. They pushed me to my limits."
Now, McCarty works out five times a week. He runs four times every week, a total of at least 20 miles, and does weights twice a week. He's also a personal trainer at Downsize Fitness, helping others reach their goals.
To help his physical transformation, McCarty made drastic changes to his eating habits.
"Before, my range of food was: Hamburger, pizza, something fried," McCarty said. Now, processed foods are strictly forbidden. He eats a lot of fruits and vegetables, grilled chicken and grilled fish.
When he drinks sweet tea, he dilutes his serving to 75% water. Of course, he still splurges on one guilty pleasure now and then: The Oreo Cookies Blizzard at Dairy Queen.
The journey hasn't always been easy. During his running and training earlier this year, he tore a meniscus in his left knee.
"I was scared to death -- if I couldn't work out, the old me would pop back out," he said.
His surgery was on a Thursday. By Monday, he was back at the gym, working on his upper body, chest and shoulders.
Two weeks later, he was walking on the treadmills. At three weeks, he was able to jog, and by the fourth week, he could run again.
Beyond the numbers on the scale, McCarty has seen many benefits to his weight loss.
Remember how he agonized over a running a mile in 22 minutes in August 2012? In June, he ran a 5K race (3.1 miles) in under 28 minutes.
After he crossed the finish line and saw his time, "I just started crying," he remembers. "I just kept walking and thinking about where I came from."
At his job, he's noticed a change as well. He's always had to be on his feet all day, but because of his weight, he used to avoid tasks involving standing on ladders and leaning or reaching -- he was scared he would fall.
"Now, I'm like a little ninja up there," he said. "I can reach over, and I don't care about falling no more."
Weight loss contests at Downsize Fitness proved lucrative for McCarty. He originally had no intention of winning the first round, from September to December, but when he learned he was close, he stepped up his workout to four hours a day, five times a week. That earned him $7,000.
The January to June phase, which came with a $20,000 prize, was an incentive to keep going with his weight-loss efforts. After winning, he paid off his truck and bought his wife a nice camera; the rest is in savings.
He has since kept off the weight, but the gym won't be doing more of these contests, a spokeswoman said.
McCarty has several more goals to keep himself motivated.
Right now, he's training for a half-marathon that takes place September 14, and in October he'll do a sprint triathlon. His goal for next year is a half Ironman.
He also wants to help the rest of his family get fit. McCarty has issued a challenge: They're all going to run a 5K together in December.
"I don't want to be a hypocrite to my kids and saying, 'You've got to lose weight now,' " he said. "But I am going to challenge them to do this one thing for me. I think they're going to like the results."
Next month, the family is going back to the same amusement park -- Silver Dollar City in Missouri -- where McCarty had to give up his seat on a roller coaster last summer.
Trying to buckle the seat belt then led to unexpected twists and turns in his life. Now, he's ready to experience the full ride.
Do you have a weight loss success story? Share it on iReport