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.