For me, the key to prepping for a campaign is keeping a solid core of information that you use to guide you as events develop forward.
Broadly speaking, all I think you need to prepare three main things:
-The “Big Bad” –the oppositional force, the enemy, the villain. What do they want and why do they want it? How do these desires lead them into conflict with the heroes? And what happens if the heroes do nothing? Make the your bad guy strong, give them a clear motivation and well defined perspective.
-The “Seed” -the core facts upon which you build your story, or you might say: “what’s really going on." Ideally, the seed is not so big as to bog you down with details, but full of crucial information that you can use to build outward where you need to build out (if A is true and B is true, therefore C, D, and E must also be true) Keep the underlying information succinct and dynamic.
-A Sense of the "Hooks" (and also "Pins") – In the "Hero's Journey" sense, this might be called "The Call to Adventure". The Hook involves how the story "appears" from the outside and how it pulls the characters in. A Hook is like a stray thread (or threads) that, if you pull hard enough, could unravel the entire proverbial sweater (and potentially tangle you up in yarn!!!) I would offer a "Pin" as a twin to "Hooks" - Pins are the way a story deals with characters that ignore or fail to deal with problems.
These three areas comprise a dynamic core that will keep you afloat as you navigate the story forward.