As excited and eager as I am, I have made a conscious decision to now throttle back a bit on the creative torrent that's been pouring out of my brain on it. There are lots of reasons for this, not the least of which is that everything I put on paper now is something I'll have to either refresh my memory of later, or re-create to fit the situation as it has evolved by the time the players get to it. Another is DM burnout: there is a finite amount of work that will be done for any campaign, and if I "use it all up" now, there won't be more for later, so to speak.
But finally, and probably most important, I have been trying to move to a more "sandbox" style, which means that the players need to be in the driver's seat when it comes to deciding what actually happens in the campaign. But the more I prep, and the further out I prep, the harder it will be for me not to push them in my "favorite" direction, whatever direction that happens to be. If I spend a lot of time now figuring out the detailed machinations of the [spoiler], that will all be effort wasted if the PCs decide they'd much rather go figure out the mystery of the strange [spoilers] down in [spoiler]– and vice-versa.
So right now I've got broad brush-strokes, and that's enough. From here I pretty much stick to prepping just as far as the next session, although in practice I'll probably add more bits to the megadungeon part on a regular basis, at least until it's got all of the major chunks roughed out and detailed far enough for the players to get one session in. My weekly prep then becomes deciding how the world reacted to what the PCs did last week, reviewing the maps/stat blocks for whatever chunk the players are likely to interact with this week, plus a bit of time spent tying it to their characters and adding a little touch of awesome. Long-term prep can be reviewed again after some more sessions are actually completed.