Running high mileage is the foundation of a training cycle. It does not make you fast, it prepares you for the training that does make you fast. A typical cycle would look like 12 weeks of mileage, gradually building to what you can reasonably handle. Then a hill phase (10 x a big hill), a few weeks, a tempo/fartlek phase of a few weeks, then speed a speed phase (6x800 with 3 mins rest).
In the hill/tempo/tempo/fartlek/speed periods, you want to be working out about three times per week and going for easy runs about 4 times per week. The easy runs should be EASY pace. Number one problem I have seen is people not taking easy days easy enough. You want to be ready to kick some ass on the hard days. If you work too much on easy days, that detracts from what you can do on hard days.
Any part of the this cycle alone is not really useful, combined they are great.