From the always good NY Times health and fitness section.
I had my left knee 'scoped over seven years ago (deteriorating meniscus needed to be cut back, so I lost about 60% of the knee cushioning on the inner aspect) and yes it bothers me. Sometimes quite a bit. Several months ago I stopped walking on the treadmill (used to go 4 - 4.5 mph) and now just use the ellyptical machine (and occasionally the concept 2 rower) in the gym That has helped.
Unfortunately, knee osteoarthritis, a condition characterized by a slow, steady loss of the knee’s cushioning cartilage, is progressive and, at the moment, incurable, says Dr. Michael Parks, an orthopedic surgeon at the Hospital for Special Surgery in New York City, who specializes in knee replacements.
But there are ways to slow the progression. “The most important,” he says, “is activity modification.” While long-distance running, for instance, doesn’t seem to directly cause arthritis in most people’s knees, it may speed the condition once it starts, Dr. Parks says. “It’s best, once you have a diagnosis of knee arthritis, to limit repetitive impacts on the joint,” he says. So switch from running to cycling or low-impact elliptical training. Swimming is also good, he says, although the breaststroke, with its whippy frog kick, may stress the knee and should be avoided. “Water aerobics are also very good, especially for people who are older or overweight,” Dr. Parks says. “The buoyancy of the water unloads the knee.”
The whole article is good, and only totals seven paragraphs.