When you think about it, a GM and an author are quite similar. Both are telling a story. The only difference is the medium and the audience.
Of the two, running a game as a GM might be the harder option. As a GM you still have to spin a story, create believable characters, and pace the plot but you also have to react – on the fly – to the participating players.
As a writer, I am quite fond of going back and tweaking the way scene play out. In theory (I’m moving there everything works in theory) I have an unlimited amount of time to make things perfect. As a GM, I have one chance to get it right.
True, a huge amount of setting and background work has been done for me. Especially if I buy a lot of sourcebooks. (Or borrow them from a good friend – you know who you are). Even so, a GM is working on a story in real time with his (or her) audience. A lot of prep work goes into effortless looking spontaneous reactions. Ask you GM sometime how much material they have created that was then never used.