Becoming Faster doesn’t mean always running fast. Why, you may ask?
Too much speed-work in your base build will interrupt your fitness development.
Endurance training (below your lactate threshold) helps you grow more mitochondria which provide energy for distance running. This allows you to become faster.
While it is true that speedwork will get you faster, you will plateau right away if you do not force yourself to slow down and take it easy on a regular basis. You will exhaust your system causing your racing performance to decline and workouts to suffer.
Speed-work does produce great benefits such as, increasing your VO2max, helping your fast twitch muscle fibers to perform more efficiently, as well as improving your lactic acid tolerance. We want to do only as much speedwork as needed to achieve the desired results; avoid overtraining. This is where recovery runs come into play. Aerobic training (easy/recovery runs) will teach your body to utilize oxygen, preserve glycogen stores by using fat for fuel, helping you become more efficient. Easy runs help you to recover faster allowing you to give a hard effort on the hard days. This keeps you from becoming chronically fatigued.
It is easier said than done to slow down. Run these by feel, you don’t always have to use your watch. I have found that always looking at your watch all the time, subconsciously makes you speed up. Try not taking your watch on your easy run days; if you are exhausted, red in the face, and short of breath at the end, you ran too fast, slow it down next time.