The following knee problems are very frequent in long-distance runners: 'runner's knee' (patellofemoral pain), IT band syndrome (iliotibial band friction syndrome), and 'jumper's knee' (patellar tendinopathy). One of the best ways to treat these injuries is to strengthen your knees with the 'step down' exercise.
Here is a very good exercise for the treatment of patellar tendinopathy. This strengthening exercise can be done as much as twice a day, for 3 sets of 20-40 repetitions if symptoms allow to. Make the exercise more difficult by adding weights or stepping down a higher step. This exercise is recommended for patellofemoral pain, iliotibial band syndrome and patellar tendinopathy.