My usual sprint routine consists of ten 100-yard sprints. I'll sprint, walk back to the start, then turn around and sprint again - rinse and repeat.
Unfortunately, the local high school football field I usually sprint at was being used by a spring soccer league. I went to another field complex in the next town over from me. The 2 fields here were both being used by another soccer league. Dammit.
These fields have a decently steep hill next to them, so I decided I'd run hill sprints. I'd never done them before.
I managed to run 4 sprints, and I was completely wiped out. My legs were like Jello, and I was breathing way harder than I do during my normal sprint workout. Very tough workout. I'd highly recommend adding hill sprints to your routine if you can.
"Don't waste your time, or time will waste you."