Hills that take about 90 seconds to climb are great for running economy workouts. The grade should be less than for hill sprints and short hills. Your effort should be at about mile race pace; 8 to 10 reps is a good target. Because you’re now climbing a fairly lengthy hill, the jog down should give you plenty of time to recover. If, however, you get to the bottom and aren’t yet ready to go hard again, jog around a bit more. Hitting the proper intensity is much more important than simply being out of breath the whole time.