Iliotibial band related pain is one possible cause of pain on the outside of the knee in distance runners. Other structures can cause similar symptoms, so the first job is to work out what is actually being irritated.
Note when the pain begins, whether hills or pace alter it, and what happened to your training beforehand. Rehabilitation may involve temporarily changing the runs that provoke it while building relevant strength and gradually testing the return. Simply stretching the outside of the thigh is unlikely to explain the whole picture.
WHEN TO GET HELP
Seek assessment if the knee swells, catches or gives way, or the pain is not following an expected pattern.
Further reading
This is general information, not a diagnosis or an individual rehabilitation plan. An assessment may change the advice that is right for you.