Not sure if this will help anyone but when I first started running 4 years ago I had sore/stiff knees. It hurt just above and to the side of each kneecap and my physio said the muscles on either side of the kneecap weren't holding up (and as it shifted it hurt) so she suggested plies. I skipped ballet as a kid but I found the exercises really helped strengthen the knees and no more pain. So for any other ballet ignoramuses who this might help: heels touching, feet at 90 degrees, knees in line with toes, hold your body in a long line and centred above your pelvis (no leaning forward/backward) and gently flex your knees out/toward the floor, without letting your heels come off the ground. Keep your knees in line with your toes the whole time, as you lower and then raise again. Hope that makes sense.
I quit running for 2 years (way too hot in Australia) but kept the plies up intermittently and have had no problems since starting running again this year.