"No plan survives contact with the enemy" is a good piece of wisdom that is almost universally applicable. In RPG terms the joke version is normally "no plan survives contact with the players" and it can be true. The fact is, you never know where your players are going to go. All signs might point to them going to location A, only for you to sit at the table and the players to head for Location Z. Alternatively, maybe you expect something to be done quick, but the players decide to linger. That is also alright.
The key thing to remember when it comes to your prep is that that is all it is, preparation. You prepare the things you feel are likely to come up. However, that doesn't mean it is going to happen. And that is ok. This is why Mark Hulmes in the video I linked to on Friday prepares broadly around where his PCs are. You just don't know if something is going to click oddly for the PCs and send them off in a random direction.
Of late I've caught myself doing two things that are unfair to myself and my game. The first is killing myself trying to prepare more content than I could possibly ever use or need for one session to be ready. This is unfair to me because it puts undue stress on me, and also puts unreasonable expectations. This is also bad for the game because working to unreasonable expectations means I'm invested in those things...meaning they're more likely to happen without change.
The second thing is that I've felt obligated to get PCs through some parts of my notes. Not the whole thing, because that is fine. However, I feel I've been bad with pacing. Sometimes the players give signs they want a slower session and pushing forward to the next thing isn't good there. And sometimes the players are focused and driven, and trying to slow things down there can be just as bad.
In my case I feel the second is in part because of the first, but they can happen in either order or in isolation.
Ultimately my goal should be to run the best session I can every session. And to do that, the goal should be to run the best scene I can at any point we're in a scene. There are other things around that - balancing spotlight time, hitting the right notes, etc, - but the basic idea is to not fret about how far you are in your notes. Handle the scene in front of you. Let the next one come as it can. If the PCs don't get through all your prep, great, half your prep for next session is already done. If they do? Great, you know exactly where you need to start fleshing things out.
Relax. Have fun with it. You're not on a timer.