This is a great question! I enjoy planning for long-term developments too, and it took me a while to get into the session-method.
Separate your prep into two types: (1) macro-level campaign prep to stimulate the GM brain, and (2) micro-level session prep for this week. And do not - I repeat DO NOT - cross the streams.
When you’re in a good mood and want to plan for larger campaign developments, do that and only that. When you’re getting ready to run your game in four days on Saturday, do that and only that.
These two types of GM prep can dovetail into each other, but resist the urge to try and do both. You won’t have time for either. Of course, if your campaign hasn’t actually started yet you have an “infinite” amount of time - do as you please.
I want to start a new campaign. I’m excited about the idea of bandits, tribes of orcs and related kin, and want to explore the dangers of ancient sorcery. Now I know a lot about my campaign via extrapolation.
Bandits are running wild. Why? Maybe the king is old and senile, or being beguiled by his court mage(!). I like this second one, and it ties into my third point of interest. We can both spend ages figuring out who the bandits are, what they’re doing, where they are, and the different dynamics of their lives of crime.
There are tribes of monstrous humanoids (orcs, goblins, hobgoblins, and so on). Where do they come from? Well they are spawned in the deep slimey places of the world, in the halls of the dead dwarves. Who is breeding them? A powerful magic-user seems likely, but maybe too obvious, so I opt for illithid who want to use them as a slave race.
Ancient sorcery exists, and is dangerous. There are huge questions here… Can the PCs use magic? Is “magic” different from “ancient sorcery”? I decided there are court wizards, so there is probably two different tiers (maybe in AD&D terms, first through 3rd level are common, 4th level and higher are ancient sorcery, etc.). This will be a lot of my macro-level prep probably. I get to think about things like okay, if the teleport spell exists, how does it work? Maybe they need to use ancient “star-gates” scattered around the world as foci (you can teleport, but only to and from these gates). If the fireball spell exists, maybe magic-users have to learn it from fire giants or demons. Now we can start thinking about cults, religions, and all of that tangled mess.
In short, you probably have some things you’re excited about doing in your campaign. Jot them down, and then spread out like cracked glass. You don’t have to answer everything - write down questions you have about the campaign too.
It’s session one this weekend. Let’s presume we’ve made characters and are ready to go on that end. I want to show the players something about my world in the session, and suggest some paths for them to pursue thereafter.
(1) Intro: some description and introduction, must include a situation that they need to solve (combat, escape confinement, find someone or something, prevent violent from erupting via diplomacy, and so on)
(2) Branching Paths: once they’ve completed the first scene, give them a branching path. One maybe is the STR path that warrior types will excel at. Another is the INT, WIS, or CHA path, that your “support types” will be good at. Another is maybe DEX or CON.
What’s a STR path? Fighting, intimidation, or hard work
What’s a DEX path? Sneaking, stealing, or least resistance
What’s a CON path? Long travel, enduring, or holding breath
What’s an INT path? Deducing, deciphering, or researching
What’s a WIS path? Empathizing, intuiting, or engaging other life (animals)
What’s a CHA path? Convincing, fast-talking, or seducing
(3) Funneling Back: their past success should give them different advantages (avoid the combat, fight the combat but reduce enemies later, etc.) for the next one. This is maybe a set-piece encounter, and I often use combat here. It isn’t for everyone, but I enjoy slug-fests where folks can “play the game”.
(4) Development: Something happens, something goes wrong, a new complication appears. You’ve made it to the old tower where the orcs have taken up. But they have bolstered it’s defenses and repaired it. The old bridge over the ravine is ruined, and you’ll need to find another way in. Oh, and it’s swarming with orcs. If you don’t make it in fast enough, they’re going to bring those missing merchants down, via the tunnels they dug, to the illithid lair to be eaten.
(5) Culmination/Climax: the session end, big fight or big reveal, success or failure. Most of the narrative here tends to exist from playing it out.
(5.2) Now what: have ideas of where to go now; back to town to recover, to the old library in the abbey for research, to a nearby town to return those merchants you saved or tell their families you failed, etc. What you suggest here will greatly help your prep for the next session. If you suggest nothing, and nothing is obvious, players may flounder at the question of “What do you want to do?” - it’s a blank page. Listen to what the players want to do and prepare for that.
As with all the above advice, session prep the ICRPG way is all about having fun NOW! Not putting it off ten sessions down the line. Thing about all the GM advice in the book. Active environments, timers, treats, wonder, and so on. Use them, but don’t over-saturate by using all of them all the time.
TLDR: You got this. Just do the thing and ask the community for help if you draw a blank.