What Causes Chronic Pain In Knee Radiating Down The Leg?

I have one leg that is aching. Started off with the knee being sore a week ago and now my lower leg is aching almost continually. The sore knee has happened before but has gone away after a few days which is not happening this time.
I did fly for 3 and a half hours before the knee got sore this time, got up once during flight. Went on a 3 hour walk a few days ago and this did not make it any worse