How do i deal with knee pain after running long distance?

Workout safely: You can apply ice over the knee, do a little bit of compression with an elastic bandage and apply cold compress over the site of pain. If this occurs after strenuous exercise this might be normal, if this condition happens all the time it is important to attend with a physician to discard any other problem like osteoarthritis (though you are young for this condition). Workout safely. :)