In 2007, Legg went back to work and rejoined the SEALs as a platoon commander. He knew his struggle wasn't over, but he was managing the disease.

Managing it, that is, until a doctor made a mistake with his medication, causing his symptoms to flare up again. His hands and feet itched like crazy. A mouth full of sores caused any crunchy food to hurt like a razor blade. Cause for more concern was his liver and lungs, which were failing rapidly.

By February 2009, it had gotten so bad that he had to resign from his SEAL team. "That crushed me," he says. "That was everything I had worked for since I was 14 or 15 years old."

And the worst was yet to come.

"I remember the day my lungs collapsed. I remember them putting me on a gurney, calling a code blue, rushing me through the hospital."

Legg slipped into a coma.

"The doctors told me how sick he was, how close to death he was," Suzanne says. "I was just thinking ... there is no way Justin can die like this. He will be so PO-ed."

After two weeks, Legg awakened from his coma. For a while, his body seemed OK, but doctors were worried about his chest scans. After everything he had been through, the news that he would need a double lung transplant barely fazed him.

"Death wasn't an option," Legg says. "In my mind, I've always thought that I have the ability to prevent this from happening. I have the ability to fight it off."

He had the operation on July 22, 2010.

Back on his feet

Just eight months after his transplant, Legg completed a half marathon. In September 2012, he half-ran, half-walked his second.

Working with Team in Training, a fundraising program with the Leukemia & Lymphoma Society, Legg has collected more than $39,000 this year for cancer research. He hopes to raise another $11,000 by running the Disney Wine & Dine half marathon in November.

If he reaches his fundraising goal before the end of the year, he'll be allowed to name a research project after his lung donor, 19-year-old Jarred McKinley Carter.

"Jarred abruptly ended his life on July 20, 2010, without knowing what an incredible person he was," his mother, Julie McCarthy, told CNN in an email. "Jarred died believing that he didn't matter, that no one would remember his name... Justin and Jarred are now joined together -- a brotherhood -- on a mission to help others. They endure training together, go on adventures together. My son still lives and breathes."

Legg hopes to meet McCarthy in Florida after the November race. He is reminded every day that he wouldn't be alive if it weren't for her son.

Legg still struggles to do simple tasks, like mow the lawn or climb stairs. Most lung transplant patients die from complications within five years, a statistic that's always in the back of Suzanne's mind.

"I constantly ask him, 'Are you OK? Are you breathing?' That drives him crazy," she says with a laugh. "(But) if there's one person that can live for 25 years with a new set of lungs, it's going to be him."

For now, the couple is focusing on taking things one step at a time -- all the way to the finish line. Legg strives to follow his SEAL team's motto: Earn Your Trident Everyday.

"You must do something to improve yourself each day," he wrote on his blog. "That doesn't mean just doing what is required. That means stretch your limits a little bit farther than the day before."