Also I think its worth noting how much less depth the US has in the marathon. In the late 1970s/early 1980s the US had over 200 sub 220 marathoners with one year having 230 I believe. In 2006 there were only 23. I don't know about more recent figures since I couldn't find any, but the point is that the US has dropped off in terms of depth at the elite level. And when you look at the overall average time, in 1980 the average American marathon time was 3:32, in 2011 that figure dropped to 4:16. Now only have we fallen off at the elite level but also at the non-elite level as well.
I think the major reason for this is attitude. At the elite level, now we focus so much more on quality and moderate mileage and scientifically based training. Sure, this may help some of the super elites(Ritz, Hall, etc) run faster than the top guys from Rodgers' era(Hall and Ritz 20458 and 20747 compared to Salazar Rodgers and Shorter 20813 20927 and 21030), but we lack the depth. Some of us just don't have the talent or resources(high altitude machines, underwater treadmills, sleeping 12 hours a day, etc) to train in such a structured long term based way. In the Rodgers' era everyone just worked hard. If you were a serious runner you ran anywhere from 120-150 mpw with the most serious guys routinely putting in 140 mpw. The increased depth at the time is testament to how this approach helps produce a large number of very good runners. It may not reach 100% of your potential, but 99.9% of the time it'll get a heck of a lot more than being a pansy and taking the careful approach. The careful approach has caused the US to fall off as a serious contender on the world stage. The US is absolutely pathetic on the world stage now compared to the Ethiopians and Kenyans in the marathon. In 2006 when we had 23 sub 220 marathoners, Kenya had over 500. Considering the US population is about 5 times larger than Kenya, we have no excuse. They're not more talented or anything, if you tell yourself they are you're just making an excuse. Kenya beats us because they work hard! They're not afraid to go out and put in some serious work! They don't know the science behind the training. They just train and do what their coach says will make them fast. Some are self coached (like 20342 man Wilson Kipsang and 20302 man Geoffrey Mutai), but this way they listen to their bodies. Rodgers and Shorter didn't know jack about the science behind the training, they just worked hard. Shorter did 120-170 mpw, sometimes as many as 3 runs a day, 20 mile long runs every Sunday, and twice weekly speed workouts. No science showed that that was the best way to train, he just did it because it produced results. If it wasn't for that doper Cierpinski, Frank would be a two time Olympic gold medalist. And frankly, until America is willing to throw caution to the wind and train like the guys of old, we are not going to challenge the Kenyans and Ethiopians in the marathon. When the overall depth of your country is better, it raises the bar. If America was to have 500-600 sub 220 men like Kenya, then to stand out you have to be sub 205 guy, so even if nothing is done differently in training, just the mental aspect of all those guys elevates your game.