Don't know if I could keep at 175 for 30 minutes, it was all I could do to keep it between 175 & 185 for 1 mile this past Sunday... Guess I'll have to incorporate your interval workout schedule into my skating
The point is to work at the highest intensity level that you can for the duration of your exercise. If you can only maintain 150 bpm for 30 minutes and feel like you can't go any further then that's the best you can do. If you want to train longer you'd need to reduce the intensity.