This is one of my favorite workouts because it can be done in an hour or less, but still provides great benefit. It's one of the hardest workouts I'll post because you don't get much rest. I did this workout a bunch when training for the cross country ride and other century rides.

1. 15 Min: Warmup
2. 2 Min: Zone 4 (80-90% of Max heart rate) You should feel a burning in your legs, but this isn't an all out effort.
3. 1 Min: Zone 5 (91-100% of max heart rate) This should be an all out effort. Don't leave anything behind.
4. Repeat steps 2 & 3 three times.
5. 5-10 Min: Easy peddling
6. 2 Min: Zone 4 (80-90% of Max heart rate) You should feel a burning in your legs, but this isn't an all out effort.
7. 1 Min: Zone 5 (91-100% of max heart rate) This should be an all out effort. Don't leave anything behind.
8. Repeat steps 6 & 7 three times.
9. 15 Min: Cooldown 

You can shorten this workout by decreasing the repetitions to two times instead of three or decrease your recovery time in between sets. I wouldn't cut it below 5 min though.