Another nice read on this topic is this Scientific American article from awhile back: The expert mind (Ross, August 2006)- "Experts are made, not born."

It's more enjoyable with the original typesetting and art work, but the text is freely available here.

It makes the same point of working on problems at the right level:

"Ericsson argues that what matters is not experience per se but "effortful study," which entails continually tackling challenges that lie just beyond one's competence. That is why it is possible for enthusiasts to spend tens of thousands of hours playing chess or golf or a musical instrument without ever advancing beyond the amateur level and why a properly trained student can overtake them in a relatively short time. It is interesting to note that time spent playing chess, even in tournaments, appears to contribute less than such study to a player's progress; the main training value of such games is to point up weaknesses for future study.

Even the novice engages in effortful study at first, which is why beginners so often improve rapidly in playing golf, say, or in driving a car. But having reached an acceptable performance--for instance, keeping up with one's golf buddies or passing a driver's exam--most people relax. Their performance then becomes automatic and therefore impervious to further improvement. In contrast, experts-in-training keep the lid of their mind's box open all the time, so that they can inspect, criticize and augment its contents and thereby approach the standard set by leaders in their fields."