Tough workout, we did some tempo work, which I think is tough on the indoor trainer, maintaining a solid tempo effort at zone 3 is harder than it would seem.  We tend to ride at either zone 2 or zone 4 with a zone 1 recovery.  But 3-4 minutes at tempo builds solid high aerobic efforts, especially taxing on the endurance and thus builds overall muscular endurance.  

I find that athletes in class comment that their legs give out before their heart rate - meaning that they can work at a higher HR, but their legs start failing first.  These hard intervals at a solid zone 3 on the bike work on that failure.

Try it on your own, after a solid warmup, do 3-4 min intervals at zone 3 (using enough resistance, at 90+ rpm)  on 20-30 sec recovery.  It will help you build your overall muscular endurance!

Ali