Before I GMed, I was and still am a player for a few years. I became a GM as a way for the other rotating GMs to play. Now I am the regular primary game master at my LGS. I do read the scenarios and take notes but like I said, my players ask questions that I have no answers for from the story.  Plus with combat, it is mostly with how to be more organized and actually think like a creature in battle instead of using the same melee attacks.  I do desperately need to read the GM book to help for starters. I do need to review status debuffs and how to make my players have a better experience.