Once a week i do hill repeats. Hill repeats are a great way to build strength and speed especially if you are short on time. I can find a shady hill and complete them any time during hot summer months. In the winter, look for a sunny hill or one protected from wind by houses and trees.
Preferred hill is about 5- 7% grade and takes from 1:30 to 2:00 to climb. I always descend each repeat with the first one being very easy. If at any time I do not improve or descend from the previous repeat - the work out is over. I strive for 5-6 of these repeats. But if #5 is slower is than #4 I pack it in and move on to the cool down. Warm up and cool down is always a mile or 8 to 10 min of easy running. I then do some leg swings in all directions prior to the first repeat.