Playing short form requires several shifts in technique and approach. My group typically does 2-3 hour sessions. A 4 hour session is a marathon for us.

Drop the Filler

The first thing to do is let go of filler material. Filler material includes setting up adventures that are "clue to clue to clue to oh actually interesting development". This is the default for a lot of play, and it wastes time. Let go of having the players play out haggling for everything, or explain each store they go to for supplies, and so on.

If there are encounters that would take up time but not actually provide interesting choices or hard implications? Cut them out.

There's also a lot of time wasting in putting in situations that make it unclear what the players should even be trying to do. "Where should we go next?" "I guess we keep searching until we find a clue of what to do next?" etc. Videogames used to use mazes to add extra hours of gameplay as a filler device, and this is the same kind of thing.

Let players get to the fun interesting stuff without this obfuscation and play not only goes quicker, it's more fun period.

Scene Framing

Scene framing is the next step. You no longer say, "Ok, where do you want to go next?" as the default question. If you know the general thing the players are doing, you skip up to the next part and get right into it.

For example, you know the players are trying to track down a cult:

"After three days of talking with shady people, listening to wacked out babbling of street preachers, and having to do some small trading in hallucinagens, you finally find out where they're having meetings. At dusk, you find yourselves outside an abandoned church where several people died of plague years ago..."

Think of how movies or tv shows will give you a montage - so you can skip the legwork and go RIGHT to the interesting stuff. This alone cuts out a LOT of time wasting material.

The Rules you use matter

If you're playing a game where combats take 45 minutes... well, one combat will take up a significant chunk of your time. Understand that many of the older games and traditional games expect you to be playing for 6-8 hours as session, so if they expect 5 combats in a session, and you've got 3 hours to play... it's not going to fit.

If you're playing a game where conflicts of all types, take 5 minutes? Well, then things move much quicker. So pay attention to what rules you're using and what their impact is, so you can plan appropriately.

Getting into character, exploring the world, etc.

I've been playing this way for several years now. My group gets into characters quite well, because the focus of our play is on characters, their issues and personalities, which is because a lot of our game time is putting the characters into crisis points and fun interactions.

I find a lot of people used to playing with lots of time, think the solution to character or world development is simply pouring on more time, when in fact, it's about directed and focused play. I've had several people say, "We've done more in this two hour one shot than I've done in entire campaigns, for years."

That's because I cut out the filler, focus on the characters' choices and actions, and try to give situations that are at turning points. Each scene should matter and have some impact.

Take good notes and remember for the players.

1. Take good notes. Don't rely on your memory for what happened. Instead, take notes during the game and then allot about an hour for going over and adding to those notes. Basically what you want to have when you're done and packing away your notes is a little kit of information that will tell you everything you need to know to keep running the game. Expect to completely forget anything you don't write down, and that will make sure that you have useful notes. Treat it like you're preparing notes for another DM to pick up where you left off!

   Then, weeks or months later when it's time to prepare for the next session, you can take those notes out and read them as the “background” for the next adventure, as if you weren't even there.

2. Remember for the players. During the game, be your players' memories. When you bring in an NPC that they previously interacted with, make that part of their reintroduction. (E.g., “A tough-looking woman with a broadsword on her hip enters the tavern. She's Meredith, the mercenary that you worked with briefly but parted on poor terms with. She hasn't noticed you and walks over to the bar…”)

   By being generous with this information, you make up for what the players are missing: immediate recall of what just happened last session.

These two things are all you really need to make a session work after a long time. Provided you have players who want to be gaming (i.e., enthusiasm waning due to long breaks isn't an additional issue), these practical steps all that's necessary to patch the time gap.