Hi Piwacket. I've been told that running downhill is very hard on the body.
Varying the pace is the basis of what are called interval workouts, and it is supposed to be better for you to do that. A 10-min interval workout might be 3 min warm up, 1 min at fast pace, 2 min slow, 1 min fast, 3 min slow. It is supposed to be better for you and increase your metabolism. There is recent research that a couple of 10 min interval workouts a week is better than longer workouts:
The two-minute workout works wonders: Short, sharp bursts of exercise are just as good as a 90 minute run | Mail Online