“The biggest predictor of injury is previous injury,” -NY Times article on running and knee problems
...by moving and loading your knee joint, as you do when walking or running, you “condition” your cartilage to the load. It grows accustomed to those particular movements. You can run for miles, decades, a lifetime, without harming it. But if this exquisite balance is disturbed, usually by an injury, the loading mechanisms shift, the moving parts of the knee are no longer in their accustomed alignment and a “degenerative pathway” seems to open.
MY COMMENT: This is good news if you've been running as long as I have (counting high school I ran my first race in 1968!) and have not suffered any significant knee problems.