Let's just begin by stating that you do not need to run slower ALL THE TIME. Unless you are working on building your base mileage, you can and should be adding faster runs (i.e. speedwork) into your training routine. Basically, neither slow long runs or fast short runs stand alone.
OK, back to why it works. The ultra simplified reason is that slow long runs increase your endurance and increased endurance is the key to running faster. Basically, slow running allows you to run MORE (often with less chance of injury than fast running) and running more now and having a stronger base helps you to run faster later.
But, there are more benefits than that:
- The thought is that running too hard every day will cause over training. The body needs time to get stronger and adapt to the stresses you place upon it. By slowing down, we aid the body in adapting and recovering.
- Slow running improves your aerobic system, which also improves your ability to run harder paces.
- Running at a slower pace will help improve your capillary system which improves the rate at which blood gets to your muscles when you do run fast.
- Slow running trains your body to use fat as a fuel helping to spare your glycogen stores.
- Slow running simulates race duration, building your mental strength.
- You recover quicker from an "easier" effort, allowing you to have a better quality workout when you do speed up.
Did I convince you to try it? I know, probably not. Slowing down is a hard pill to swallow. But really, what's the worst that can happen?