By BETHANY LYTTLE David Shack’s Type 1 diabetes had been so out of control for so many years that he had had more than 100 seizures. The highway patrol once stopped him at a roadblock because he was weaving.

“My eyes were acting up, and my blood sugar levels were so low, they thought I was a drunk driver,”said Mr.Shack, 31, a science teacher and father of three in Boone, North Carolina.

So, of course, he decided to participate in an Ironman race.

Given that Mr.Shack had gone long stretches avoiding medical care and enjoyed nibbling on steak fat or chicken skin, he made an unlikely candidate.

“Dave was a doctor’s worst nightmare,”said John Moore, who lives in Denver and sells annuities.“There’s no question his life was at risk.”

Mr.Shack was recruited for the 2008 Ford Ironman Wisconsin by Mr.Moore, 31, who also has Type 1 diabetes. Participants would have to follow a 3.8-kilometer swim with a 180-kilometer bike ride and a 42-kilometer run.

But, Mr.Shack said,“Something about the craziness of it got me going.”

And go he did.

Mr.Shack completed the race in September. It took him about 16 hours.“I was the last dude across the finish line,”he said.

Faced with a chronic condition or a terminal diagnosis, some individuals start training regimens that even the healthiest would find taxing. And the result is a fascinating if somewhat incongruous equation: people fighting sickness or disease who are, at the same time, in the best shape of their lives.

“It’s not always as simple as some sort of headlong rush into denial or a desire for supreme control,”said Dr.Gail Saltz, a psychiatrist at New York-Presbyterian Hospital/Weill Cornell Medical Center.“People who have a close brush with their own mortality sometimes experience a reaction akin to separation anxiety. A separation that, in this case, is from life.”

Some may push themselves too hard, while others may put too much faith in their new physicality, believing that they can defeat disease and sickness purely through physical exertion. But experiencing the body as capable, experts say, can be a way of choosing to own (rather than disown) the body. And this can be a powerful, and empowering, way to cope.

Kim Klein, a student at Loyola University Chicago School of Law, experienced such a shift. Until 2007, she had managed to avoid exercise for most of her 41 years.

“I was the person who was always going to join a gym and never did,”Ms.Klein said. And she hated running. Then came breast cancer. Ms.Klein, a mother of three, decided to train for a five-kilometer run.

“Something had changed,”she said.“I was desperate to feel my body again. I needed to know it was still there.”

In April last year, she finished that race? and went on to complete four more.

“Look,”she said,“there’s no question that I’m running away? yes, literally? from the cancer. Every step feels like maybe I’m farther away, maybe I’m stopping it from coming back.”

But for some, the realization that they can’t outrun an illness can be devastating. Richard Brodsky, 56, found out he had terminal brain cancer in 2002. Increasing his running distances each day, Mr.Brodsky, an architect and philanthropist, set out to run the 2003 New York City Marathon.

“It felt so right to me to be pushing myself,”said Mr.Brodsky of Atlantic Beach, Long Island, in New York.“Then a radiologist told me that fitness had nothing to do with my life expectancy, nothing at all. It was such a blow. I guess on some level I knew it, but when I heard it I got very upset.”

Still, one year and one day after learning of his diagnosis, he crossed the finish line with his neurologist at his side. For medical professionals, patients like these present a challenge. Just because it is exercise doesn’t mean it’s good.

“A line can be crossed,”said Christine Mermier, an exercise physiologist at the University of New Mexico at Albuquerque. “However, my experience is that patients of this type tend to be very in touch with how much they can get away with. Those who develop a real passion for athletics seem to find a way to walk that tightrope.”