Very interesting Outside Magazine article reviewing a very large scale study, almost 90,000 runners and walkers over 7 years. The runners had half the risk of osteoarthritis and half the hip replacements when compared to walkers, regardless of mileage.
While risk of osteoarthritis increases with age the cause in runners is usually from other sports (tennis, skiing, soccer, basketball and even from deep knee bends at the gym) really any other sport than running. Running promotes cartilage thickening. Runners also tend to be leaner which obviously helps.
"While all physical activity should theoretically provide some protection for your joints, Williams says he was surprised to find that forms of exercise other than running and walking actually boost your risk of osteoarthritis. In fact, people who were most active in activities other than running were at a 21 percent greater risk for osteoarthritis and 99 percent increased risk for hip replacement compared to those who exercised the least."
So, as my high school track coach used to say: "Don't play games, you'll get hurt. Just run."