The New York City marathon is 37-year-old Cris Dopher's Mount Everest.
It's even more remarkable considering Dopher's lung capacity is 30 percent less than normal.
"Maybe my body isn't built for marathon distances," Dopher told CBS News contributor Dr. Sanjay Gupta. "But to me, I need a goal."
So while he's often forced to walk to catch his breath, he's determined to train.
"It feels really good at those moments just to be able to run free," he said.
Dopher was just 4-years-old when he was diagnosed with cystic fibrosis, a genetic disease defined by a steady decline in lung function.
"Most cystic fibrosis patients were kids," he said. "And would die as kids."
In fact, when Dopher had a breathing attack in college, he could only find a pediatrician to treat him.
"Adult clinics were few and far between," he said. "There just weren't enough adults."
That's because most patients died before they reached adulthood.
From the 1950s to 1980s kids diagnosed with cystic fibrosis barely made it out of their teens. Now, the average life expectancy is 37.
Key reason why? Experimental drugs that don't just treat infection and inflammation, but target the disease itself.
They catch the disease early on and prevent or significantly delay lung disease. These drugs are very promising.
Exercise is another reason Dopher is still alive. Now he's on course to run his second marathon with one goal.
"Finish," he said. "It's going to be a tough race."
On marathon day, Dopher was struggling, walking much of the way. By mile 23, he was forcing himself to keep going.
After 5 hours, 50 minutes and 12 seconds, he crossed the finish line, revived by memories of friends who'd lost their fight to CF.
"In the last 8 miles I tried to remember it's more about them than me," Dopher said.
Eventually, Dopher will need a lung transplant. He knows his next five or 10 years may be his last - but he's not giving up.
"After finishing New York, I just ... I could do that in less time, I really do!" he said.
Still, he's grateful for every breath it took to cross the finish line.
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