(CNN) -- His friends used to call him "Hoss," as in Hoss Cartwright from the TV show "Bonanza." At 6'2" and roughly 380 pounds, Mark Rucker once resembled the big, friendly character.
Now, they call him "The Beast."
The nickname is apt. Since February 2011, Rucker has run his first 5K, his first 10K, his first half-marathon and his first marathon. He's completed an Olympic-distance triathlon and lost more than 100 pounds. And on August 26, he'll participate in his first full-length Ironman competition in Louisville, Kentucky.
Like they say, he's a beast.
"What he does after this Ironman, I don't know," his wife, Annita Rucker, said with a laugh. "I hope he can find something to focus all this energy on."
From Rucker's DailyMile blog: April 18, 2011 -- "First day of the Couch to 5K program. That was not good. But at least I did the whole workout" (1.98 miles/ 32 minutes/ 16:10 pace)
Change is a funny thing, Rucker says. Often, the desire to change grows over time. A series of events leads to an "a ha moment," when desire must meet with the proper tools and the right inspiration.
In the summer of 2010, Rucker, of Lexington, Kentucky, and his family went to Dollywood in Tennessee. His daughter was 11 at the time, a "little daredevil," and wanted to ride all the roller coasters. Unfortunately, Rucker couldn't fit on any of them with her.
"She was devastated; I was crushed," Rucker said. "But even that wasn't enough to get me started on the journey. I guess I just didn't know how to do it."
In January 2011, he reconnected with an old elementary school friend, Anita Mills, and the two began talking on Facebook, discussing Mills' 240-pound weight loss.
Her transformation was his inspiration. But he still wasn't ready to change.
After weighing in at about 365 pounds, he stopped stepping on the scale. He was diagnosed with sleep apnea and put on medication for high blood pressure. He feared that he was diabetic but wouldn't visit his doctor.
The day after the 2011 Super Bowl, Rucker woke up in pain. Once again, he had overindulged the night before, and his digestive system revolted. He was tired, sick and unhappy.
"Something clicked," he said. "I realized that if I kept doing it, I would significantly shorten my life."
Rucker took Mills' dieting hints to heart and cut out sugary drinks. He dropped his two-liter-a-day soda habit and picked up water. He stopped ordering McDonald's McGriddles for breakfast and eating peanut M&Ms by the pound. He threw away his Cheez-Its.
"It's hard to say what my daily (calorie) intake was," he said, looking back. "I did it to myself. I was really unhappy with the way my life was, and I think food was that comfort for me."
June 19, 2011 -- "Last day of Couch 2 5K. I think I'll call it The Father's Day Beastly 5K. I was the only competitor and I finished in 1st place. I can't even begin to explain how I feel right now. And no one or no thing can ever take this away from me. I AM a runner. I AM an achiever. I AM a Beast. On to Bridge 2 10K!!!"
Rucker's job as a real estate attorney kept him sedentary. So he started to walk on his lunch break, using his iPhone to post his distance on DailyMile.com. The site shared his progress on Twitter and Facebook. Friends posted replies that the site's app read aloud to Rucker through his headphones during workouts.
"That was a huge motivator to me," he said. "Even though I was by myself, I still had people to encourage me."
He ran his first official 5K race, the Great Buffalo Chase, on July 4, 2011, in 39 minutes. Less than a month later, he tackled a 10K, finishing dead last but finishing all the same. His blog that night complained about the cross-country course's hills but promised to begin training for a half-marathon the next day.
"The focus he has, the drive he has ... it's so refreshing," Annita Rucker said. "It's the old Mark I didn't even know -- probably the Mark from law school."
The Ruckers married in October 2000. For 11 years, Annita Rucker watched the man she married -- the dreamer, the achiever -- struggle to maintain his optimism while grappling with a shrinking housing market and a growing body.
Their son and daughter -- two children from Rucker's first marriage -- came to visit often, and Annita Rucker found herself entertaining the young kids while her husband remained stationary.
"I used to complain that he was living life on the sidelines, never participating," she said. "Now, he's so active with them. Last year, my father had Parkinson's and was hospitalized for 10 weeks. Mark stepped up to the plate and did all the cooking, the laundry, everything. Before this, that never would have happened."
"My wife jokes, 'I've got a brand new husband,' " Rucker said with a long laugh. "I think she's a lot happier with me ... because I'm much happier."
August 18, 2011 -- "Three mile walk at work on my lunch break. Some days I gotta dig down deep to find the motivation. Some days it's really hard. Today is one of those days."
Rucker is now training for his first Ironman competition. A typical week includes 4:30 a.m. swims, 6-mile runs after work and hours of cycling on Saturday and Sunday. At 260 pounds, he's stopped focusing on losing weight and started focusing on building muscle.
There are good and bad days. Some blog posts sing of the serotonin rushing through his brain. Others show his discouragement of a bad swim or a slow run. He's a beast, but one who feels pain.
Still, it's amazing, even to him, that a guy who less than two years ago got winded walking up a flight of stairs can push himself this far, this fast.
"I'm the guy in the overalls (who's) shoveling the coal in the train," he said. "I don't know exactly where it's going, but I'm going to do everything I can to keep it moving."
February 18, 2012 -- "Woot woot!! I'm a marathoner!!" (26.4 miles/ 6 hours, 13 minutes/ 14:07 pace)
Rucker has rejoined the social world. He volunteers at the Red Cross and is involved in his local Rotary chapter. This year, his family went back to Dollywood and rode all the rides together.
Rucker wants to become a motivational speaker, to be that inspiration for others that Mills was for him. Possible title for his speech: "Be the Beast that you can be."
"When I was so overweight, I felt so dead to everything," Rucker said. "There was this little voice in my head that said, for so long, 'This is the best your life is ever going to be.' And that voice is gone. I've extinguished that voice."