Add a course expansion which has a lot of rolling hills. By this, I mean a route which consists of hills anywhere between 10 and 25 meters high that are about a 6-8% grade at steepest. There should be little if any flat between the hills. The fun thing is you can use your momentum from going down the hill to help you up a decent portion of the next hill. In some ways this landscape lends itself to natural intervals. On a group ride you can travel quite quickly over this terrain as it favors both the climbers and the heavier power riders.