You can of course plan the big lines of the campaign, but the more precise you get and far ahead of the present, the more you will either lose or railroad to not lose. Both suck
Encounters, easy. You were going to prepare those anyway.
Maps, not as easy but there's resources online in a pinch.
Forces... What do you mean by that?
Terrain... That's just maps again, right?
Ecosystem... Yeah, you're definitely over-preparing at this point.
Descriptions... You shouldn't have been prepping this anyway. If you know what the thing looks like, you can describe it yourself during the game.