Jeff Galloway recommends acceleration-glider (ACG) drills to conserve energy and thus be able to run faster. The idea is to take advantage of the momentum of your faster pace to "glide" as you gradually slow down. The gliding takes less energy since you're using the momentum from your faster pace. They would be appropriate for going down hills and for slowing down for walking breaks, water stops, etc. Jeff recommends that the drills be repeated several times once a week. He cautions, however, that we shouldn't sprint during the drills.
Let's assume you're running and want to slow to a walking break. Rather than making a relatively quick transition to walking, gradually slow down and let your momentum carry you as far as possible. By doing this, you will cover more distance with less expenditure of energy.
I've done this unintentionally in the past. I usually take about two steps to go from running to walking, but on occasion I've let myself go farther as I slowed down to a walk. I literally felt my body push me as I slowed down. I'm taking walking breaks every half mile, and I'm going to use ACG to transition to the walking. Also, my two-hour and longer runs involve a couple of large hills and a couple of small hills, and I'll use ACG to go down the hills without a lot of energy being used. Of course, I'll eventually be able to run my normal pace down the hills as my legs and knees gain strength.