"A calendar is a to-do list, and a to-do list is a calendar." Well, no, a calendar is an absolutely-have-to-do-at-that-specific-time list. I don't see how you could've gotten that idea from GTD, because David Allen recommends not using your calendar for todos that you'd like to have done today, but don't need to have done today. Conflating calendar and to do list will quickly lead to what dkarl calls the "Do this now or you're a bad person" phenomenon, and lead you to stop using todo lists altogether because they're so depressing.

The biggest win of todo lists for me (I use the GTD-esque Things) has been that it gives me some assurance that I've got everything that I have to do or want to do (even someday/maybe projects) written down and in one place, so it doesn't soak up cognitive energy throughout the day that would be better spent just doing things.