Increasing speed and endurance is a key factor to enhanced performance. And while there are many ways to increase your speed and endurance, hill training and interval training are two of the most effective methods.
Running fast is a simple formula—it’s a combination of your "stride length" (the distance between your feet) and your "turn over" (the frequency of your steps). Increasing either, or ideally both, will increase your speed.
Hill training is excellent for improving your leg strength, which will help you increase the distance you cover with each stride, thus increasing your stride length. Hills also require you to pick your knees up higher when you’re running, which also is a way to help increase your stride length. As we fatigue, our knees tend to drop, and we don’t get as much push off of each leg. Therefore, hill training helps train the leg muscles (especially the hamstrings) to combat the effects of fatigue.
Running up a hill also effectively uses your body as a weight. Start with a hill that takes you 30-60 seconds to run up and has an incline to where you feel fatigued at the top but not so steep so that you can't run up. Run up and jog down slowly for your recovery, then do it again. I usually do sets of 10 with my clients, or at least 20 minutes. For sprinters, I do shorter, steeper hills and for distance runners, I do longer hills at a lower incline.
Interval training is similar to hill training. Interval training helps improve your turnover and is great for building your endurance.
The idea is to train the body to go faster in shorter spurts with the hope that it conditions the body to move quicker so that when you are racing, it will feel natural to move the legs faster. (Downhill running can also be useful for this.)
With intervals, you should plan to go fast for a short period of time and slow for a period of time, and this varies based on the race you’re running. For example, if you are trying to break a 5:30 mile (11mph) then you may want to do 400-meter intervals. You could break it into one 400-meter interval at 12-13mph pace, then one 400-meter interval at 6mph pace and alternate between the two. Do that for a total of two miles. Slowing down and speeding up places more stress on the heart, which also helps the runner get into even greater conditioning.