There came a moment in the hospital just after Jim Clapp's left lung was removed when he was ambushed by a new, unexpected threat to his life: a severe allergic reaction to a transfusion of blood platelets. His heart raced dangerously. He gasped desperately for breath.

Clapp knew he was facing a showdown with cancer, but he wouldn't let it take him like this. It was a cheap shot, he thought. He got angry. And he got resolute. "It was like someone smacking me in the face. It got my back up."

There came a similar defining moment two years later, last July, as he jogged along a bicycle path through Arlington County. He was in his 13th mile, a mile longer than he had ever run during the three months he had been training for a marathon. Suddenly, the endeavor no longer seemed fanciful.

"I said, damn it, I can do this. I can do this now."

Today, 31 months after he learned he had lung cancer and was told he had at most 30 months to live, Clapp will compete in the Marine Corps Marathon. Running on the breath of a single asthmatic lung, the 42-year-old former construction contractor knows he will not finish near the front. When he completes the race, if he completes it, Clapp expects the race organizers will already have taken down the finish line.

But his wife, Anne Hicks, and his own victory will still be waiting for him beside the Iwo Jima Memorial in Arlington.

"I'm not doing this to qualify for the Boston Marathon. I'm just doing this to complete the marathon," he said. As he continued the thought, his voice cracked: "If a guy with one lung can do this, it might really show other people with lung cancer that they don't die immediately. Just maybe if I can pull this off, it can give hope to someone who is in the depths of hopelessness that is cancer therapy."

Clapp had been an athlete in high school--a typical jock, as he recalled--but never a serious runner.

And though he remained fit as an adult, he developed a hacking cough, hardly unusual for a smoker like Clapp. When he had it checked, he initially was told that he had asthma and should quit smoking. He followed the advice, but the cough didn't clear up. The next diagnosis was either pneumonia or acute bronchitis. Chest X-rays, however, revealed the truth: a tumor on his left lung. The illness was so far advanced that doctors told Clapp he might have as little as six months to live and suggested that surgery might be pointless.

It was after the chemotherapy to shrink the tumor and after the operation, as Clapp was taking walks to drop the pounds he had added, that he first tried to jog around the block near his home in Arlington. In the following weeks, the block became two blocks and then a mile and then an increasingly ambitious program of early morning runs.

Training for a marathon, Clapp said, has become the perfect metaphor for battling lung cancer: the intense physical strain, the dreadful prospect that it all might come to nought.

"I remember being barely able to get out of bed and pull up my socks and go for another chemotherapy session," he said. "I was able to see past the immediate horror. But there's a lot of uncertainty when you deal with cancer."

More recently, it's been the grueling regimen of waking up at 5 a.m., tugging on his running shoes and setting out on the bicycle paths, unsure whether he will ultimately ever be able to finish the marathon. "I decided I'll do what it takes no matter how bad it may be."

Clapp has learned to pace himself, to run until he feels short on air and then slow to a walk until he regains his breath. He believes he can finish the marathon if he keeps the adrenaline and the competitive edge in check, if he doesn't strike too fast a pace. He figures he can complete the race in seven or eight hours. (The winning time at the Marine Corps Marathon last year was just under two hours and 26 minutes.)

If Clapp doesn't finish, it won't be cancer that stops him. The latest word from the doctors is that he's clean.

"I'm nervous. I've never trained this hard for something. There's a little bit more on the line," he said, noting that he's gained a following among others fighting lung cancer. "If I have to walk off the course and not finish, I'll be disappointed as hell. But I won't be a failure."

CAPTION: "Just maybe if I can pull this off, it can give hope to someone who is in the depths of hopelessness that is cancer therapy," Jim Clapp says of running the Marine Corps Marathon.