"Where you may get some advantage in high mileage--if you can stand it--is in the substrate, the metabolic level that relates to glycogen storage. With volume over 100 miles, you're depleting yourself on a chronic basis and forcing yourself to replenish your glycogen stores day after day."
"If you can push the depletion level of the body in races from 60-90 minutes to two hours by training, you can maintain a higher intensity, which translates to a better running speed. That is where volume mileage has the advantage over high intensity training." -Al Claremont
William J. Fink of Ball State University suspects Claremont's assumption may be correct, but also suggests that volume training may result in a neuromuscular training effect, a more efficient recruitment of all available muscle fibers, which allows the work load to be parceled out over the distance more effectively: "When a runner doubles his training mileage, we often see no change in his maximum oxygen uptake (max VO2), his ability to deliver oxygen to the muscles. So we are forced to look to other areas to determine why volume training results in better performance." (Note: Bill Fink was head of the Biochemistry room in the Human Performance Lab at Ball State- I spent 2 years in there doing whatever I could "not to screw something up." He definitely knew his stuff, especially with a Ph D in Theology!)
"If you're talking about the reasons high mileage results in fast times, I'm not sure we know what we're looking for yet. Most scientists probably have not zeroed in on the real causes." -Jack Wilmore
article by Hal Higdon for ultRunr.com
MY COMMENT: 100 mile weeks are common among world class marathoners. I did it in college and for a short period after I graduated. I clearly remember feeling pretty beat up by Wednesday, however just one easy day was all I needed to bounce back and feel great. How fast you run those miles is also a consideration. 100 mile weeks will leave you feeling very strong after a taper, however to sacrifice quality for the sake of a reaching a mileage goal is not the best idea for running a fast marathon. You need to find a way to inject some quality in those weekly miles in the form of intervals or sustained tempo runs. Finding an ideal mix of distance and quality is the dilema all serious marathon runners must face.