The 20 Minute Hill Workout

Min 1: Treadmill @ 3.5mph @ 1.0% incline
(The First 3 minutes is a steady paced walk)
Min 4: Treadmill @ 3.5mph @ 2% incline
Min 5: Treadmill @ 3.5mph @ 3% incline 
Min 6: Treadmill @ 3.5mph @ 4% incline
Min 7: Treadmill @ 3.5mph @ 5% incline
Min 8: Treadmill @ 3.5mph @ 6% incline
Min 9: Treadmill @ 3.5mph @ 7% incline
Min 10: Treadmill @ 3.5mph @ 8% incline
Min 11: Treadmill @ 3.5mph @ 9% incline
Min 12: Treadmill @ 3.5mph @ 10% incline
Min 13: Treadmill @ 3.5mph @ 11% incline
Min 14: Treadmill @ 3.5mph @ 12% incline
Min 15: Treadmill @ 3.5mph @ 13% incline
Min 16: Treadmill @ 3.5mph @ 14% incline 
Min 17: Treadmill @ 3.5mph @ 15% incline
Min 18-20: Cool down. Treadmill returns to 1% incline at 3.5mph. 

MODIFICATION TO MAKE EASIER: Make your walking pace 3.0mph.

MODIFICATION TO MAKE HARDER: After you've reached the 15% incline, go back down 1% every minute until the treadmill returns to 1% incline. (It's the reverse of what you just did). 

FUN FACT: This treadmill workout is adapted from a test that firefighters have to perform for their annual stress test. With a doctor present, electrodes are placed in specific locations on their chest to get an EKG report of how their heart reacts to exercise. Often times, this test is performed on a Super Treadmill which reaches a 40% incline. This allows the firefighter to exercise until fatigue which creates a baseline for returning years.