Competitive runner Steve Woo, who logs as many as 10 hours at the gym and 50 miles on outdoor trails per week while training for marathons, gets asked all the time: Are you addicted to fitness?

“Since I was a kid, I’ve always been told that I run or exercise too much,” said Woo, 32, of San Francisco, who has thighs the width of small tree trunks and calves as hard as rocks and has been running since he was a teenager.

Woo’s weekly exercise regimen includes a mixture of running; doing squats, lunges and calf raises; and using the elliptical trainer, a machine that mimics running.

He’s so obsessed with making sure he gets his fitness fix that before he takes a vacation, he has to find out whether there will be a running trail or a marathon in the town he will be visiting.

He ran his first marathon (26.2 miles) when he was 15.

“By now, the ‘You’re overdoing it’ spiel goes in one ear and quickly out the other,” said Woo, who is 5 feet 9 and weighs 145 to 155 pounds, depending on his training. “Exercise and working out have always been as routine as brushing my teeth, yet no one says I’m an addict for brushing my teeth two or three times a day. Some people call my running an addiction. I call it a passion. It’s a healthy addiction.”

But Woo has had serious health consequences from his passion. He has had to have surgery on his left knee because of mountain climbing and operations on both calves because his muscles got so big they began constricting the nerves in his lower legs.

Sports medicine doctors and fitness trainers say that more and more people like Woo are pushing themselves too hard and that their bodies are paying the price. They’re hooked on the rush of endorphins — chemicals in the brain that decrease pain and generate a sense of well-being — commonly known as a “runner’s high,” which some people experience after exercise. Others have body image problems.

With extreme sports gaining popularity and more people seeking to have the six-pack abdominal muscles of models on the covers of magazines and on TV, experts say it’s no wonder some people have developed an unhealthy dependence on exercise.

“There’s no question people get addicted to fitness, just like people get addicted to narcotics,” said Dr. Warren King, an orthopedic surgeon at the Palo Alto Medical Foundation and a team physician for the Oakland Raiders. “Endorphins are the body’s natural heroin.”

King said he’s seen patients who have suffered severe injuries and put themselves at risk for premature aging — bone and joint deterioration is a common result of over-exercising. Other patients, he said, have sacrificed relationships and jobs for their workouts. He’s referred several patients to counseling, though most have declined.

King said exercise junkies run the gamut from triathletes and ultra-marathon runners to type-A personalities and narcissists who ignore pain (which becomes especially acute in the weight-bearing joints of the knees, ankles and hips) and proceed with the same intense routine.

The most common long-term consequences, King said: deteriorated cartilage and joints, which can lead to debilitating arthritis.

“The body is only meant to take a certain amount of use,” said King, himself an avid runner. “Our bodies wear out over time, then we aren’t able to do the activities like exercise that we love.”

Philip Ha, 16, of Oakland, Calif., gets hassled by his friends and his older brother for over-exercising. Ha, who works out about two hours every day on a weight machine, said some of the criticism is deserved, although he doesn’t consider himself a fitness addict.

He has injured his arms from doing too many arm curls and has had to sit out a couple of games at Oakland High School, where he plays outfield and third base on the varsity baseball team.

“My friends tell me I should stop before I look like a freak,” said Ha, who is 5 feet 6 and weighs 145 pounds. “I do try to lift heavier weights, build more muscle and push myself because I’m insecure and trying to get that perfect body.”

Woo, despite his proclivity for pushing himself too hard, said he tries to maintain balance.

“I’ve learned to train smarter and to listen to my body if it’s screaming for me to stop,” he said.