WASHINGTON, D.C. ? Hobbled by aching knees and an inflamed right calf, America’s first runner has been all but sidelined from his favorite exercise and says his woes serve up a good lesson to all sports-minded baby boomers.

“Listen to your body. I tried to run through the pain and it didn’t work,” President Bush said Thursday.

The president first complained to a reporter about his knees during a private party in late April in Washington, blaming it on age. Bush is 56.

On Thursday, the president gave a public tip off to what he called “a runner’s injury” on the tarmac at Andrews Air Force Base in suburban Maryland as he walked toward Air Force One, before traveling to Minnesota.

Playfully, he abruptly clutched his left knee with both hands, then broke into a wide grin, leaving most observers puzzled.

More than two months ago, Bush, an avid runner for more than 15 years, noticed an aching right calf muscle after a hard run.

It turned out to be a minor muscle tear, White House press secretary Ari Fleischer said, and Bush reined in his running about that time.

Bush consulted his physician, Air Force Col. Richard J. Tubb, about the injury, Fleischer said.

A minor muscle tear and aching knees have hampered President Bush's regular running regimen. Bush playfully clutched his leg Thursday as he was escorted to Air Force One at Andrews Air Force Base.

Amid the tensions of the Iraq war, Bush had pushed himself to perhaps the best times of his life, clocking a three-mile run at an average of 6 minutes, 45 seconds a mile a strong pace for a runner of any age.

A fierce competitor, Bush almost always times himself “with an eye on ‘Can I beat it?'” Fleischer said.

Partial tears can be treated with either rest or heel lifts, which are cushioned pads inserted into shoes to relax the calf muscles, said Dr. Jacob E. Tauber, a Beverly Hills, Calif., orthopedic surgeon. “A more significant tear could actually require casting. No surgery is necessary if it is a partial muscular tear.”

Despite his injuries, the president is hardly idled.

He has increased other forms of exercise, including lifting weights and swimming less jarring activities that allow his running injuries to heal. He continues to exercise six days a week.

“This is a good lesson for people in their 50s who want to remain athletic,” Bush said, adding: “I believe I’ll be running again.”

Indeed, the president recently tested his running, but at “a more realistic pace.”

In his last time out, a week ago, Bush clocked in at 9 minutes a mile.