I think that sounds like too high for too long. You might talk to the instructor. The good ones will provide some information about this. Yes, when I first started I would blow up at some point, and woudl have to slow down. For me, the trick is a sufficient warm up, then you can do a lot more without blowing up. When it does start, you should definitely back off. I agree, it gets 'better but not easier' because you compensate by cranking it up more. For me, it does definitely help, especially with sprints and hill work.
The bicycle is the most civilized conveyance known to man. Other forms of transport grow daily more nightmarish. Only the bicycle remains pure in heart. ~Iris Murdoch, The Red and the Green