But yes, if you have your adventurers going from dungeon to dungeon, adventure to adventure, they might advance to high levels in a very short period of game-time. You can either A) run your campaign so this doesn't happen, and thereby protect your sensibilities, or B) change your sensebilities by changing your idea of what "high level" really means.
In other words, if a really lucky, brave, unusually competent and highly motivated individual can go from levels 1 to 20 in about 2 game years, what does that logically mean for your campaign world?[/B]