You should feel it mostly in the glute of the front leg. Make sure you are pushing through the heel of the front foot, and that the knee of the front foot isn't too far forward (try to keep it over your heel). If you are feeling it in the back leg, you might be using that leg to push too much. Try placing your foot flat, top of foot down on top of the step, to reduce help from the back leg (it should just be there for balance). Or, try putting that foot on a stability ball or a med ball instead of a step.
For back arching, definitely try using a shorter step height, and keep your core braced and strong.