Hills - more precisely running down them, just did 15.5ishkm as I've the half marathon in a couple of weeks and I'm very ill prepared. Just done a night run and going up Airlie Road was fine but coming down was a killer on the knees.
Best of luck with that. Sadly my surgeon says I have to give up anything that put stress on my knees. Football and golf being two of my loves, havegone by the board. Eventually a knee replacement will happen.