Most GMs I know tend to prepare session to session. It's a little like living paycheck to paycheck, just for your RPG. Yes, you may know the general overarching plan of what is supposed to go on. You may know the NPCs, their motives, etc. However, you don't have the specifics for a session planned until the days before that session happens.
There is absolutely nothing wrong with this.
The nitty gritty details of session preparation is best done before the session. How else are you supposed to know what happened in the previous session you need to follow up on, or have a read on the general mood and events going on? If you plan a session several weeks in advance, you could have any number of variables that will throw that plan out of wack. So why put in the work if you are just going to have to redo it?
My answer to that is the benefits you can get with the ability to do a second pass on the material.
For example, for my previous session of one of my games I planned out a whole beginning for the adventure the PCs are setting out on. The PCs then decided they wanted a shopping session, and ran around a city buying goods and getting set up. We ended the session with the PCs just arriving where I expected the session to more or less begin. Which is great with how weird schedules are right now, because it more or less means my prep for the next session is already done.
So what am I doing this week to prepare?
Review The Material
I'm going back over everything I have for this session. Reviewing my notes on NPCs I made quick sketches for, locations, setup, and other hitches. With the time apart from when I wrote it I more or less have a fresh perspective. This will hopefully help me find any places where things are a little janky, or otherwise don't make complete sense on what is going on. Anywhere I find I pause and go "wait, what?" is a place I need to massage material.
Identify Places for Depth
When preparing for a session I don't go too deep into most things. That way they can come out in play and I have room to adapt to what is going on with my players. With a second pass I can look for places that I think would benefit from having more depth in them from the getgo and see about adding it in. The idea here isn't to go off writing a book and wasting prep time - we still only want to prep what we need not want - but having some ideas for how things could be deeper is a great idea.
Review Obvious Methods Of Dealing With Obstacles
Planning a solution to a situation is a trap as far as I'm concerned. You get too attached to a set solution and ignore other possible solutions in play because you're not looking to see what the players do, but looking for the players to find the solution. However, that doesn't mean you can't sketch out a venues of approach. Having sketched ideas for basic approaches gives you a launchpad when the players try to do that.
With this, and looking at problems, I can give some time to thinking about different approaches. What do I do if the Bard tries to seduce the leader of the combat? What if the PCs try to bribe them? Intimidate them? Just sneak around? Run away? Light the whole place on fire? Summon a giant eldritch horror? Pretend to be bad guys?
These are all things I am comfortable improving in the moment for the most part, but having the vague sketches of a direction to go with helps with that improvisation a whole lot.
How Can I Use My Environment?
Finally, I look at any maps I am planning to have or use and try to think of fun ways I can use the environment. How would the NPCs use it? How could the PCs use it? Can I add anything to help with that?
In Superhero games I love throwing people through walls as it adds some dynamism to the fight scene and generally lets people feel pretty badass when they keep fighting after going through a wall or two. In fantasy games you can't do it as much but you can still find fun ways to do things. So take alook, and have fun.
All of this can be done on pass 1
To be clear, all of this could be done during the first pass of preparation, but having the time/distance from then for a second pass is also a big help. Either way you can have a fun session, but when you get that time to help refine it can also help. It may also end up that you don't see much need to change things or that you can't really add to it. Sometimes a simple idea is best, so why complicate it?