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Lt. Justin Legg knows a thing or two about endurance.

When the retired Navy SEAL runs in a half-marathon Sunday in Virginia Beach, every step will represent a triumph over the leukemia that nearly killed him, and every breath will be drawn by a set of lungs from the 19-year-old man who inspires him, even in death. Legg, who received the double-lung transplant just two years ago knows he won't set a personal best, but he won't quit, either. For him, obstacles exist to be overcome.

“I believe firmly that we should do something every day that keeps us challenged,” Legg, 34, told FoxNews.com as he prepared for the race. “For me it’s important to overcome.”

Legg spent a decade in the elite military unit and was in Iraq six years ago when he was diagnosed with the dreaded disease. Over the next several years, he endured endless rounds of chemotherapy and radiation treatment. His body rejected a bone marrow transplant, causing the new blood cells to attack his skin, liver, lungs and eyes. Finally, both of Legg's lungs collapsed and he slipped into a coma in the summer of 2010. When he awoke, not only did he have to learn to walk all over again, he found out he had only months to live if he didn't get the life-saving transplant.

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He did, and eight months later, Legg walked a half-marathon.

“I was hurt, but I finished,” he recalled.

Months after that feat, Legg still did not know who was behind the donation, as organ donors are kept confidential. Then he got an email that gave renewed focus to his recovery.

"I was just sitting at home playing on my laptop and a little message popped up. It said, 'Hi my name is Julie McCarthy, I'm the mother of your lung donor.'"

The relationship that ensued prompted Legg, who now trains with the Leukemia & Lymphoma Society's Team In Training, to raise his goals and redouble his efforts. When he takes on the 13.1-mile Rock 'n' Roll Half-Marathon, he'll do so in honor of McCarthy's son, Jared McKinley Carter. His goal is to inspire people to donate to a fund he set up honoring Carter. If he can raise $50,000, the Society will name a research grant in honor of McKinley.

"Jarred Carter died and in doing so saved my life," Legg wrote recently on his blog. "If we can all come together to raise this money and name a project after him, he will in turn, be helping to save the lives of so many more."

Legg won't hit the eight-minute mile pace he once exhibited in full marathons. He still finds himself winded after walking up stairs, carrying heavy groceries or, as he describes, mowing the lawn as his body adapts to the new lungs and battles the side effects of drugs that help prevent rejection.

“Needless to say, I have been cut down a notch or two - I have been as humbled physically as anyone can be,” Legg said. “In fact, many simple tasks in my life are now greatly complicated by my current condition.”

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But he'll get through it, Legg vowed.

“I’m going to switch between running a quarter mile to walking a half. The last three miles will be the worst for me physically,” Legg said. “I feel great and I’m mentally prepared for this.”

Legg’s said his tenacity predates his days as a SEAL.

“My time as a SEAL defiantly sharpened it, but by parents were the ones who instilled it in me,” he said. “They always told me not to ever let anyone tell me I’m not capable of doing something when I was growing up.”

Even joining the Special Forces unit once known as "frogmen."

“It sounded like a fun job. It was like playing G.I. Joe for the rest of your life,” he said. “Even while I was going through that rough training, I kept saying to myself, ‘I’m getting paid for this!’”

Earlier this year he attempted to climb Mt. McKinley even after doctors advised him not to. He was able to complete climb half way despite the limitations from a lung transplant.

“I feel that I can show people that anything is damn near possible, even when everyone says that it’s impossible.”