Personally, I would guess the typical runner doesn't establish a big enough base to reach their racing peak. The Runner's World-type training program just doesn't have enough mileage and has too many days off/cross-training to allow most runners to reach their true potential. Everyone is different in how many miles they can handle, but it seems lots of people try to "gimmick" their way to fast times, rather than establishing a true base. I don't think speedwork does much good until a nice base is in place. My most common advice to new runners is to just run as many miles as you can, day-in and day-out, and the rest will take care of itself.