Did these Monday and my knees are sore...
try running on a track instead of pavement
Quit leg extensions.
Made all the difference in my knee
Hell yeah. I tossed those in the bushes during Physical Therapy a few years ago. My PT explained how that motion basically presses right on your knees and that she's personally had over a dozen clients return to her after second surgeries due to them tearing or re-tearing their ACL's/ ACL reconstructed grafts completely off from that machine.
-P-
I've seen and talked to a lot of PT professionals and trainers and they say that leg extensions are one of the worst movements for your body. They place an inordinate amount of pressure on the tendons and ligaments, especially lowering the weight.
Replace them with hack squats if you have the machine at your gym.