Is it bad for my knee joints to run 15-25 miles a week. how many miles a week is too much? i'm 21 and also do free weights and strength training

Your Health: Motivation must come from you. Think about why you want to train & make that your goal. Start out small & realistic. Do something you like rather than doing a killer workout that you dread. Once you start doing that activity, even if its only a few minutes a day, you'll probably find yourself looking forward to doing it because of how great it makes you feel. Good luck!

Not too much.: You are running an appropriate amount of miles, as long as your knees are not bothering you or swelling. If your knees become painful, or you see swelling/fluid on the knee(s), then you should see a sports medicine knee specialist.