Find a location where you can pick different points on the hill. Lamp posts are great. So for example you would run to the first lamppost and back to the bottom of the hill. Then to the second lamppost and back. Keep making the reps longer until you hit 20 mins, and then start making the reps shorter again.

Focus on effort level rather than pace or heart rate as both will fluctuate depending if you are going up or down the hill