I am 42 years old, about 240 lbs, and play a lot of doubles tennis (and some singles). Anyhow, I have had some pain in my left knee for the last 2 or 3 days. The pain is similar to pain i got about 16 years ago when I trained for a marathon and increased my mileage too rapidly, and had to skip the marathon that year (but ran it the next year). Of course, 16 years ago, i was in my 20's and weighed around 170 lbs. Any advise on what I should do to prevent the knee pain from developing into something more serious? Thanks!