These kind of sessions really make me curious. Don't people get tired, or weary during such a long time? Do they manage to keep immersion and actively RP their character? Don't they lose focus? This is especially true for DMs, who have to interpret so many NPCs, describe the world and the happenings and try to create an atmosphere and help weaving an interesting story. All of that would require a kind of focus that I or my players simply can't keep up past about 4 hours.
Abundance of combat in such a long time would also make things rather dull, at least IME.
On Saturdays when I game, I usually wake up at 8am. I clean the house before the guys come over, do a little prep if I'm DMing today, then people arrive around 3. We usually have a late lunch and get started around 4. So, at this point I've been up for 8 hours. We then game for 8-12 hours.
Sure, folks get tired, but honestly by the 4 hour mark I'm just warming up into gaming.
As far as handling multiple NPCs goes, not every NPC is a dive into a new realm of roleplaying. Unless the NPC serves a purpose in the game, the people on the street, the shopkeeps, the guards are all going to be fairly simplistic. They each have a certain amount of information (usually very little) and a low tolerance for random strangers asking them a barrage of questions. After a while they're going to tell you to go talk to the NPC who
has the information (but might be harder to get it from but more reliable).
As for the environment, I keep the details down to the essentials. It's a temperate forest with average undergrowth that does not impede your travel and the trees are tall enough to give you good shade but not enough to cause it to be "dim light" or completely block out the sky. I might mention the type of trees that you generally see (pine, redwood, etc...) but otherwise I'm happy to let the players
ask about specific things. If they don't care what the soil composition is like, I'm not going to bother describing it. Not every corner of the world is a unique and magical experience. Sometimes a hill is a hill.
I write for a hobby, I tend to verge on the verbose. When I DM, I'm there to help the players
create a collective experience, not to tell a story to them.
When it comes to playing, as above, it takes me a while to warm up and focus on the game. The first hour or so is probably highly unproductive, I
want to get into the game but my brain just isn't in game mode yet. We also take breaks, maybe hit up the store for a midnight snack As far as staying in character goes, we're not theater majors and we tend to play on the goofy side. So wise-cracking, puns and so forth are all part of play.
I also work a 10-hour shift at work during the week, so "staying focused" for 10ish hours is normal for me.
