For some reason high knees are giving me problems with back of left leg at the knee. This causes me to have to back off when I've still got more. It is very important to me not to cheat the workout. Please help?
I would not worry so much about cheating the workout, but what's going on in that knee.
Knees are a delicate affair (same goes for shoulders btw.). We need them for a lifetime.
Lots of tendons, cartilage and other stuff that is easily offended. Muscles grow at a nice rate, but the rest of the framework has to adapt to that. You opened up a dialog with your knee: You're telling it: WORK and the answer was: OK, but only so far!
When you are young it's easy to ignore it, but the older we become, the more we should listen to that.
Take it easy. It's not a sprint, it's not even a marathon. It's a looong walk.
Better listen to your body and march for a while. If it does not go away, maybe let a doctor have a look at it.
All the best.