Actually you can run some very fast times on 3 to 4 days of training. You have to run very hard and very far on the days you run and do a lot of specific cross training (eliptical or stairmaster) on the other days.
When I was 39 I went from a traditional schedule of 6-7 days a week and 60-65 miles to the above schedule where I ran 3 days a week and stairmastered another 7 hours a week.
My 3 days of running included a tempo run, an interval workout, and a race or steady long run. I ran at least 10 miles every workout and ran hard enough I needed that extra day to recover. I not only ran 5k's faster than the year before, I ran an equivelant half marathon (17:00-5k to 1:19 half)even though I never ran over 10 miles on one run.
While it is obvious that the more running you can do will make you better, if that extra pounding and repetition causes you injuries, then this program would be better for you.