I would agree 20 minutes was a long time to try and maintain just under what (I think) your instructor was trying to relay as you anaerobic threshold (85%-90% and above). So if my max is 200, theoretically if I stay under 185-190 BPM I am still working aerobically.

I don't think she was trying to have you do a 20 minute interval (if she was, she needs a makeup lesson in interval training), but rather was setting your threshold and forcing you to work at or below it to stay aerobic.

I am not sure though I agree with her technique, it was skimpy on information and could be misinterpreted. Also, it sounds like it may have been a little boring!! I hope she was doing different things during those 20 minutes!!