It is not possible to reach ultimate potential without first establishing enough of a lifetime base to make your training count when you become physically mature. Most Americans do not understand this; they try to get a quick fix in this sport, and it simply doesn't work. A runner absolutely must obtain the ability to train at 120-150 (or more) miles per week by the peak of the career in order to reach fruition. There are no exceptions to this
-Four Principles To Correct Training For Elites by John Kellogg
MY COMMENT: Pretty strong statement here, but certainly food for thought. There aren't many world class marathoners (sub 2 hr 12 min) running less than 100 miles per week.