What I did was instead of normal interval/typical speed sessions, I sprinted all out as fast as I could 5 times (rest was 1.5 minutes) 3 times a week, and the other 4 days I ran very easy. I averaged out about 55 miles a week. After the 2 months, I took a week to prepare myself for my race, meaning I did one sprint workout, 1 tempo workout and the rest of the week took it very easy.
Well, the results were that I went from a 5:02 mile to a 4:34 mile! It makes me wonder if I would have had the same result if I ran regular intervals...