The August 2006 edition of Scientific American has an interesting article on how the brain gains mastery of a subject matter.
The article focuses on the mastery of the game of chess, but extrapalates to other disciplines as well. In short: Amateurs can play any number of chess games over a period of years against one another and never significantly improve their ability, while in contrast, those that study the games of the masters can go from beginner to expert and beyond in relatively short periods of time.
It is not the amount of time that we spend practicing a discipline that matters most, but who we spend our time studying!