eveeee - I've experienced plenty of knee problems :-( But I'm still running
Not going to give any specific advice as I'm definitely not qualified, but I have found hat stretching post long runs is important (one thing that's worked well for me is cycling 5 miels home after my long runs) and that swimming the evening or day after can really help loosen up the knee again.
Definitely post more details and see if someone can provide more knowledgeable advice though.
P.S. Well done on banking the 15 miler.