You can mix it up. Often workouts involve intervals / fartleks that put you in that high area for a short time and then take you out. I myself only do two speed workouts on the track and two or three spin sessions that "mix it up."
If you can do 40 minutes at 80-95% of max, I am impressed. Really impressed.
Recall that the goal of endurance training is to make your heart stronger / body more efficient. Two girls I know - one in great shape and the other in world-class shape - were out doing some bike work. They were sticking together, yet the girl who was in great shape found her hr at about 180 while the world class girl - mother of two and a few years older -was in the 120s.
From what I can tell, getting there takes a mix of intense and long training.