This largely depends on both you and your group. Do they want a lot of random encounters? Would they prefer to just play the module out? Do you want to throw things in the mix to interfere?
Hoard of the Dragon Queen, and the Rise of Tiamat, both allow for a lot of flexibility. In addition, they use milestone levels instead of XP based levels. So encounters can be fun, but with this being your first time, I strongly recommend sticking to the books. It will give you a good idea of what to do for encounters, how to tinker with the characters, setting up a good ambush, and most importantly, it will stop you from overpowering them with hordes of magical loot or ridiculously strong enemies.
As for worrying about descriptions, my advice is: Don't. If it's not important, just keep telling them, "It's still a waterfall. Your investigation skill reveals the water falling now is different from the water falling from before. Largely due to the fact that the water can not defy gravity and return to the top of the waterfall." or something like, "You turn the rock over and over, attempting to discern anything at all from it's bleak, soulless face. Getting frustrated you even try licking the rock and singing it a sweet song. Alas, it has a heart of stone, and forever rebukes your attempts by refusing to display even the slightest emotion." Then, when the players leave the area, you should pass the investigating player a note that just has, "I'll miss you.... :(" written on it. It's always good for a laugh.
They'll learn to ask about things, and you'll learn how to be descriptive, or sneaky, in setting up your terrain while they work through ways of using it to their advantage.
A piece of side advice if this is your first time: Don't be afraid to say, "Wait. I screwed something up. We need to rewind 5 minutes." Players are generally less mad at open admissions to a mistake than they are to underhanded retcons.
You can manage a 2 to 3 hour game depending on the pace you set. See the Pacing section on page 30 of the rulebook on details for how frequently to call for pulls to accomplish a particular game time and adjust accordingly. Also see page 49 for some tips on how to split your story into acts of an appropriate duration. Essentially, you dictate the length of the game by how much content you include and how frequently you call for pulls.
The recommendation for a 4 hour game is to call for a pull every 5 minutes to ensure that 1 player is eliminated by the end of that period. If you want a 2 to 3 hour game, I would recommend trying to call for a pull slightly more frequently than that, but all that does is hasten the first tower collapse. You also have to ensure that the pace at which you drive the story is matching the feel of how dangerous the tower is. Ideally, after about the 2 hour mark you can ramp up the story threats while simultaneously demanding more frequent pulls to try to force a collapse by the time 3 hours have elapsed.
This all depends on the participation and level of comfort of your players. If it is your first session with new players, they will be getting to know the game and might be slower, especially if they have never played a storytelling game before. In this case, you should design a very short story. Basically, don't think of telling a typical 1.5 hour horror movie's worth of plot. Think of telling the equivalent of a 20-minute episode of a horror show. The players will probably drag that out. It's better to end earlier than intended on a good note and leave players anticipating the next session than to feel compelled to drag a session on way too late because the plot you had planned is not yet resolved.
I believe that the intended game time is a single session of 3 to 5 hours with 4 hours being the most commonly cited runtime in the book. However, the game supports ongoing stories across multiple sessions, and if you are willing to do multiple sessions then you can pause the game at any narratively appropriate time and resume it in another session. I have conducted a multiple session campaign that lasted several months, with sessions of 3 hours each time, and it went over very well. This might be a route to take if you can plan a bigger, more sprawling narrative that can fit into more rewarding, bite-sized sessions.
Best Answer
The rules are a bit ambiguous, but it probably has a Material Plane location
Evermeet is mentioned in three of the 5e books.
Far Traveler Background
"Far to the west" seems to imply a physical place on the same plane that the captain could give an estimate for the time to travel to.
Sword's Coast Adventurer's Guide
Evermeet is mentioned in quite a few places in this book. First it is provided as a location in the quote:
Then later it is elaborated on by a second-hand account (which references the legends):
If this account is to be believed, we still have Evermeet with a connection to the Material Plane that could likely be sailed to.
Later in the book in the Elves section we get:
...lending further credence to the idea that it is a physical destination.
Mordenkainen's Tome of Foes
This is the most recent book to lend information about Evermeet, and it says:
This reinforces the rumor the writer in Sword's Coast Adventurer's Guide mentions, but says nothing to remove the presence of a Material Plane island (or part of an island). Only that it shares a presence in other planes as well. While sailing to it involves taking advantage of some special method, it probably still has a physical route on the Material Plane to get there (even if magic gets in the way of normal sailing there).