"I love the outdoors," he says, straddling his $3,000 Trek Fuel bike. "If I'm not exercising here, I'll be fishing over there. If I'm not fishing, I'll be working with the chainsaw. I really enjoy being outside, and mountain biking is a way for me to spend a fair amount of time — four or five days a week — outdoors.

"I love exercise. Prior to learning about mountain biking, I was a jogger. And then, like a lot of baby boomers, my knees gave out. I believe that mountain biking is going to be an outlet for a lot of people my age. I'm 59, and people are going to realize you get as much aerobic exercise — if not more — on the mountain bike without being hobbled."

His observation is borne out by the experts. According to Freddie Fu of the University of Pittsburgh Center for Sports Medicine, one of the nation's leading orthopedic surgeons, boomers are suffering knee and other leg injuries brought on by years of pounding the pavement. Fu recommends that they take up soft-contact sports, such as cycling. Fu even sponsors his own cycling team.

"Riding a bicycle gives the cardiovascular benefits of running without the impact," Fu says.

Chris Carmichael, who coached Lance Armstrong to seven Tour de France wins, says the president is doing the right thing.

"He's a shining example of the benefits of having an active lifestyle," Carmichael says. "If you stay fit for all of your life, you can switch sports and also maintain a high level of performance."

Carmichael is familiar with the trend: more than half of the athletes who use his online coaching service are age 40 or older.

"The president is wise to emphasize his aerobic training: Heart disease is the leading killer of American men. That should come before resistance (weight) training."

Bush's physical exam also showed that mountain biking has been beneficial for him in several areas. His resting heart rate was 47 beats a minute — the adult male average is about 68, and pro racer Armstrong clocks in at 32.

During the Saturday ride, Bush's heart rate will average 139 beats a minute, with a maximum of 177 on one of the hill climbs. He will burn about 1,500 calories — enough to erase a McDonald's Big Mac, large fries, milk shake and apple pie.

But you won't find those items on his personal menu. He weighs 191.6 pounds, 8 pounds less than a year ago. His body fat percentage is 15.79, down from 18.25 and well below the "normal" range of 16.5-to-20.5 for his age group.

"Baby boomers who were exercise folks are beginning to realize that there's got to be a better way to get exercise than running, because we are pounding ourselves," the president says. "And I'm hurt — not terribly, I don't limp — my right knee. The doctor said, 'I can fix it for you, and then you can run again. And then I'll replace it.' And I would rather not, at the age of 60, have a knee replacement."