So I don't think in terms of a virtual world.
How can you not, unless your PCs never interact with anything except each other?
Nor in terms of encounters, adventures, and campaigns. I think in terms of sets, scenes, seasons, and series, much like a TV show. So each gaming session is an episode, which is centered around 'what do the PCs do?'
Hmmm...sounds like something vastly different, both in style and pace, from what I play/run. Here, it's almost all exploration - either of a physical site, or looking for information, or looking for clues to help solve a mystery. Combats erupt whenever something gets in our way...a frequent occurrence. It's nigh impossible to plan a session, as I have *no* idea how far they'll get.
An example: I just got done running a session this evening. It starts as the party's just finished a town break; they're all trained up and ready to go, and are setting off into the wilderness with little information to go on other than a somewhat-famous name of a warlord whose stuff they have been asked to "recover" and a vague location (a forested area a few hundred miles across held by no realm) as to where he might be found. Going in to the session, I have no preconceived notion as to whether they'll meet him tonight, find his town tonight, or even get anywhere close. So, here's what actually happened:
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1. The party get into an internal argument before leaving town. (I knew this was coming, and it actually blew over faster and calmer than I expected)
1a. They also pick up 2 new characters; one was previously in an earlier incarnation of this party, and another is brand new (so new it's still being rolled up as it joins!)
2. They leave town, and get attacked a day later by some bandits they vaguely recognize as once-peaceful villagers from the last time they were here. One killing swing from the party's main fighter and the rest of the bandits flee.
3. They find the bandits' old village, now burned to the ground and lifeless; they change course and start tracking the raiders instead of going after the warlord (this is fine, and is part of the adventure).
4. They visit a cult camp they helped set up a year or so ago and find it too has been razed to the ground, probably by the same raiders. The cult leader has survived; two characters (including the returnee that just joined) leave with him to see him safely back to town. Rest of party keep going, changing course back toward the warlord again as the raiders are too far ahead.
5. The dice are nice to the party; they spend a few days in some rather dangerous woods and meet no random encounters at all. This speeds things up immensely.
6. They find a non-ruined village and learn the raiders stopped moving for a while at the next village south. Party change course again and once more go after the raiders. (all this course changing delays them by about a day overall; no big deal)
7. Tracking the raiders leads straight to the warlord's town, where the talk is all about the titanic battle that took place 5-ish days ago in which the raiders attacked the warlord and lost (their corpses are nailed to the side of his fortress as a warning) but the warlord - a minotaur - lost a horn.
8. Party split up to gather information with varying degrees of success. They regather at their inn and exchange information (and in some cases get extremely drunk) that evening. Session ends.
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There are several places where I had very limited control of the pacing unless I wanted to ignore possibly-significant game elements.
The first is #1. From the scuttlebutt I'd been hearing from the players during the week, a bench-clearing brawl was in the offing (some of these characters really don't like each other due to past in-game events) and I was half-expecting this to go on for half the night. But cooler heads (or better moods) prevailed, so little time was spent other than some insults and barbs getting tossed around.
The next is #5. In a living breathing world, random encounters in dangerous wilderness areas are a fact of life. Had things gone differently, the session could have ended with the party still bashing around in the woods; however the party sailed through without problem. (the random encounters might also have served to inform the party, had they met one of the warlord's raiding parties and taken a captive for questioning)
And the next is #8. Information gathering in a significant town is messy enough; when several characters go off on their own to find it in the ways they know best, things potentially grind to a near-halt. But not too bad this time.
Next session they'll probably spend some unknown amount of time planning, after which I've no idea what they'll do; so that one would also be hard to plan according to the "episode" standard.
Lan-"I hail and salute any who waded through all the above"-efan