Intervals. These can do everything you want. For working on endurance, do a 10 minute warm up and then do two 10 minute intervals at around 70-80% effort/max hr (however you measure yourself) with 2-3 minutes easyin between, and then a 10 minute warmdown. For improving sprints, do a 10-15 min warm up, then do three 2 minute efforts at about 90% with a minute in between, then do 5 minutes easy, then repeat the three two minute-on/one minute-off intervals, and then a 10-15 minute warm down.

I do similar workouts on my trainer every winter/spring when I can't get out on the road, and since I hate indoors riding, I'm on it as little as I have to be. I find these hour long workouts of intervals are very helpful. Just make sure you take a couple easy days every week too, don't do intervals 6 days a week!! Good luck!