I teach 10-12 classes per week. The key for me is to not get a high-intensity workout in every class. Like others said, we are there first & foremost to coach and motivate.
A couple examples on how I handle different days:
Day 1, I teach yoga, cycling, and weights. I primarily demonstrate a pose, then walk around to cue form & alignment during yoga. In the cycling class, I ride the bike but mainly cue intensity changes–I do not push my own intensity as much. During the weights class, I lift moderate-heavy weights and make that my primary workout of the day.
Day 2, I teach dance, step, and water aerobics. I keep my dance movements low-impact and just quickly demonstrate/cue the high-impact options. I teach step with no risers (4″ bench only), or even on a Mat-Step. I then make the water aerobics class my primary workout of the day.