Robert Irwin, 34, runs in the Albany Pinebush Preserve in Guilderland, N.Y. Irwin boosted his running to 80 miles a week when he was in his 20s. It felt good, until it started feeling bad. "I really started to break down," said Irwin.

GUILDERLAND, N.Y. — Robert Irwin boosted his running to 80 miles a week when he was in his 20s. It felt good, until it started feeling bad.

"I really started to break down," said Irwin, now 34. "I got plantar fasciitis in both my feet, ended up getting bone spurs. I had a really bad hamstring that hampered me for a couple of years."

Irwin, now a chiropractor in suburban Albany, dialed down his workouts to about half that. He worked his way back up over the years to a more manageable 70 miles a week — with breaks when needed.

Irwin found out that, yes, it really is possible to exercise too much. Overzealous exercisers can run their way to stress fractures, spin their way to insomnia or even overdo it to the point their immune systems are compromised.

The danger is real for both tiptop athletes and middle-agers trying to work off the holiday paunch. Problem is, the line defining when exercise becomes risky is a blurry one. There are different thresholds for different people. But medical experts work off some general guidelines.

"The real sweet spot, as you would expect there to be in any biologic system, is around an hour a day," said Dr. Mehmet Oz, a surgeon at Columbia University and co-author of "You: The Owner's Manual." "After that, it's hard to show a great benefit."

There are no widely accepted recommendations for when adults should lay off exercising — partly because health officials are worried about Americans being too sedentary, not too active. But it's also difficult to say with precision when healthy exercise becomes unhealthy among a population that includes extremes — from triathletes to couch potatoes.

"It's so idiosyncratic, that's the tough thing about it," said Carl Foster, a professor of exercise and sports science at the University of Wisconsin-La Crosse.

William Haskell, professor at the Stanford School of Medicine, says that in general the risk of harm begins to outweigh the benefits for adults after more than an hour a day. Above an hour, it's questionable whether you're going to get much more from it, he said.

An obvious concern is increasing the chance of injuries related to wear and tear, like a muscle strain, sprained ankle or a torn cartilage. Oz notes that those sorts of injuries can be a double dose of bad news: Not only do they immediately sideline exercisers, but they can cause nagging problems decades later.

There are concerns beyond breaks and bruises, of course. Middle-aged men who suddenly ratchet up activity after years of inaction can risk a heart attack. And even hard-charging athletes will sometimes show signs of lethargy, decreased immunity or headaches. The cause of so-called overtraining syndrome among athletes is unknown, but Foster said failure to rest could play a part.

Oz's rule of thumb is that bodies don't bounce back as well after more than 12 hours of exercise a week. He said overexercise can create more oxidative stress, in which oxygen molecules called free radicals are formed that damage DNA and cells in ways that, over time, can accumulate to spur cancer.

Over time, oxidative stress has been linked to health problems, including cancer.

In general, doctors' advice is listen to your body. If you are tired or achy, take a rest. Take days off and vary the intensity of your workout. Irwin counsels runners to watch out for signs they're working out too hard, such as a resting heart rate 10 beats a minute over the normal rate.

"You have to have recovery time even if you are healthy," Irwin said. "Give yourself some time to rest."

Foster offers some specific pointers for beginners trying to burn off the holiday bulge: Cross-train and give yourself six months or so to build endurance. Respect the limitations that come with age, he said, and "don't try to be 19 the first week. "

But whatever you do, don't stop exercising.

"This is not America's big problem. The opposite is," Oz said. "And people shouldn't use this as an excuse not to exercise."