If you don't run very fast in practice, you won't be able to run very fast in races.
At the University of Copenhagen, Danish scientists studied experienced runners who had been running 60 miles a week at a fast pace. One group was told to cut their mileage in half to only 30 miles a week, but to run a series of around 50 to 100 yard dashes as fast as they could. The other group continued running 60 miles a week at a fast pace. Runners who ran fewer miles at a faster pace had a 7 percent improvement in their body's maximal ability to take in and use oxygen.
Runners who did not increase their speed in practice did not improve, even though they ran twice as many miles. Jogging slowly reduces your chance of injury, but it won't help you to run fast. You can race only as fast as you run in practice, but don't try to run fast every day. Intense exercise damages muscles. Try to run fast once or twice a week, never on consecutive days and don't run fast when your legs feel heavy or hurt.
When I trained seriously for marathon running, I thought that the runner who ran the most miles would be the best. I didn't become a great runner, but I did become an expert on injuries. My patients seldom come to me with a running injury that I haven't had.
Many top runners run more than 100 miles a week because their bodies have the genetic ability to withstand such abuse, but the vast majority of runners will never be able to run 100 miles a week without being injured frequently. Furthermore, running a lot of miles slowly will slow you in races. The ability to run fast in races depends more on how fast you run in practice than on how many miles you run each week. However, every time you run fast, your muscles are damaged and feel sore on the next day, and it takes at least 48 hours for your muscles to heal enough to allow you to run comfortably again at a very-fast pace. After every workout in which you run very fast, take the next day off or run at a much slower pace. Most top athletes plan their workouts so they run very fast only two days a week. The same principles apply to any endurance sport.
Twelfth European Track Coaches Congress Acoteias, Portugal pp10-16. RRN January, 1991