(CNN) -- Ryan "Mac" McDonald's friends and family describe him as a teddy bear: charismatic, gentle and unassuming.
The description is fitting -- the former Texas all-state offensive lineman weighed as much as a small bear in 2000 when his friends bought him a gym membership for his 25th birthday.
Not wanting to appear ungrateful, Mac went to the gym and a personal trainer promptly put him on a treadmill.
Mac, now 38, got to work, putting one foot in front of the other. But it wasn't long before he looked down and saw smoke emerging from underneath the treadmill's track. The trainer rushed in, quickly explaining that the treadmill was old, on its last leg and he would set Mac up on a better one.
Mac obediently followed the trainer to another treadmill. The track moved and Mac walked. Before another 10 minutes passed smoke again rose from underneath the belt.
"After that I knew enough to stay off the treadmills," he says. "That was another one of my excuses."
Mac would balloon to more than 500 pounds and be attacked by a life-threatening, flesh-eating bacteria before re-entering a gym with any serious intention to improve his health.
But when he did, the stay-at-home dad would lose 300 pounds in less than three years.
'He never stopped putting in the calories'
The men met when they were sophomores in high school at Liberty Christian School.
"I played tailback and he was an offensive tackle, which made me automatically like him," says Scrivner.
Mac weighed between 260 and 275 pounds when he played football and made all-state his junior and senior year. He played one year in junior college and then quit.
"(Football) wasn't life for him," says Scrivner. "His personality was being a big, nice guy that everybody loves."
Mac moved to the family farm near Wichita Falls, Texas, where people began noticing his rapid weight gain. His younger brother Clint remembers visiting and seeing a mini-fridge in the living room next to Mac's chair.
"I think he never stopped putting in the calories, even though he was no longer expending them," he says.
Mac was up to 350 pounds and taking classes at Midwestern State University in 1996 when he met his future wife, Jessica McDonald.
"He was the cool guy that had the cool apartment that everybody hung out at," she says.
A year later, Mac asked Jessica on a date. She accepted with reservations.
"I remember telling a friend, 'I don't want anything serious because I've got goals to accomplish, and he is not very healthy,'" she says.
But Mac won her over with his chivalry and charm. Jessica remembers that they were only supposed to go miniature golfing on their first date but ended up going to dinner and a movie, too.
"I remember thinking, 'Oh, mercy. I'm in deep,'" she says.
Mac's relationship with Jessica did not change his eating habits. Fast food continued to contribute to his weight gain.
He'd devour bacon double cheeseburgers from Whataburger, gobble chicken McNuggets from McDonald's and wolf down bean burritos from Taco Bell. When these off-the-chart sodium servings made him thirsty, he'd wash them down with "the nectar of the Gods," Dr Pepper.
Nothing could keep him from frequenting these eateries, even hitting the big 5-0-0 on the scale.
"That's the thing about drive-through windows," he says. "You can hide when you order five things, and they ask you if you'd like two drinks with it."
Jessica remembers going on vacation to the Biltmore Estate near Asheville, North Carolina, where she toured the mansion's sprawling gardens by herself because Mac was too embarrassed to walk around.
"I wanted to be so protective of him because people would just stare," she says. "He saw the world from the seat in the car."
In November 2009, Mac saw the world from a hospital bed when necrotizing fasciitis, a flesh-eating skin bacteria, assaulted the left side of his body.
The infection was dangerously close to his heart, forcing doctors to remove softball-size chunks of infected tissue.
Mac tried to laugh off what was happening but the situation scared him. He didn't want to die.
The low-hanging fruit method
A year later, Mac hit 530 pounds and realized he had to try to lose weight.
"There is not an 'aha' moment," he says. "It was more like, 'OK, if I'm going to try to do this, I'm going to have to do it a little bit at a time. But I better do something before I lose my family to my untimely demise.'"
Mac started "dabbling" at Scrivner's gym. Next he gave up fast food and all-you-can-eat buffets. Then stopped ordering soft drinks. Finally, he started buying healthy food from the grocery store.
Mac describes his strategy as the "low-hanging fruit" method. It allowed him to lose weight without having gastric bypass surgery, hiring a nutritionist or discovering a dieting secret folks in Hollywood would give their big toe to know about.
"It was simple, but it wasn't easy," says Mac. "I knew that I had to make a change because I was sick and tired of not being able to do anything I wanted to do."
Rachel Berman, a registered dietician and director of nutrition for Caloriecount.com, says setting small, attainable goals like the ones Mac made is a better approach to weight loss than using an "all-or-nothing mentality" because people appreciate the progress that comes from each change.
"If you do too much at once, it can be overwhelming and that makes you less likely to maintain it," she says.
Scrivner says he avoided Mac when he first started showing up at the gym, asking members to not say anything to his friend.
"We've hounded him so much over the years," he says. "We were all just kind of holding our breath and hoping he would stick with it."
After Mac lost about 50 pounds, Scrivner approached him about entering an upcoming Jailbreak race.
Together they made a video inviting people to sign up to run the approximately 3-mile obstacle course, promising Mac was going to keep exercising and enter the event, too.
True to his word, Mac competed in the Jailbreak race at 350 pounds.
"I just knew I wanted to complete it," says Mac. "I remember crossing the finish line and being ecstatic."
He has since entered and completed 5K and 10K runs, a half-marathon, a duathlon and a sprint triathlon. He will enter the Hotter 'N Hell Hundred-mile bicycle race this August.
At 230 pounds, Mac is now light enough to ride a motorcycle, an activity his weight kept him from for years. In November he will fulfill a lifelong dream by racing a motorcycle in the Baja 1000 in Baja California, Mexico.
"I had people in my life that I could go to and ask questions and I could use their knowledge," he says. "I did not do this alone."
Mac still does weekly weigh-ins and says the battle to maintain his weight will last the rest of his life. He spends his free time with Jessica and their two children, Matthew, 3 and Libby, 5. The McDonalds will celebrate their 15th wedding anniversary in December.
"I think we're stronger now than we've ever been," says Jessica, who is writing a Ph.D. dissertation about obesity's impact on the learning process. "He's my biggest cheerleader."
Mac also takes time to share his story with others struggling with obesity. He has a blog and occasionally speaks to kids at the YMCA of Metropolitan Fort Worth, encouraging them to begin healthy nutritional habits early. He recently took two 500-pound men grocery shopping, invited them to his house to show them the types of foods he now eats and gave them some of his old regular and workout clothes.
Scrivner says he believes Mac's story is better than a personal trainer for those trying to lose weight, because he has walked in their shoes.
"He's about the most inspirational person I know."