The Unexpected

Running a game is hard. What I'm finding from trying Storm King's Thunder is a new lesson in difficulty!
I love the book. I think it has wonderful options, I'm a sucker for giants, and the scale is as large as the adversaries. Unfortunately, the scale is enormous, and it has a multitude of options... and it leaves the DM and players to choose them. I knew there were three possible giant attacks on towns to choose from for the second chapter, and expected each to explain the lay of the land, then say "this is what happens when the giants attack." I'm finding there's a big difference between "the giants attack" and "this is how to run the encounter when the giants attack." I was expecting the latter, and the gave me the former.
Just to make things more exciting, I thought we'd have a setup encounter or two this week, then face the town attack at the next session... with time to prepare. Our party bypassed the setup encounters with little incident or fanfare, and there we were in town, being awakened by a screaming drunk who'd seen looters.
I thought, "this was always a possibility, I'll just look over the instructions for running the encounter, and figure out a way to make it work as we go." That's when I noticed the instructions were essentially "the giants attack." Now, there was more to it. The giants attack in three locations, treated as separate encounters, and once two of them are thwarted, the third runs. And so on. But let's get real. This battle is set inside a walled settlement with an 8-10 mile radius from the town to the walls. How do you treat for scale? How do you account for town guards and militia? Do the five character have to defeat all 6 giants, 8 bugbears and 60 goblins, or will the guards help somehow?
I pulled it out, and it seemed to go well, but it felt like it could have been more, but I just don't know how. I tried to prepare encounters for the session, and they were avoided. I expected to have directions on running the printed encounter, and there were none.
That's the essence of running a game, though. Anticipating two or three possibilities and preparing for them in as much detail as you can, so all you need to do at the table is make it happen. Then, when everything goes sideways, scramble to pull something together from thin air and expectations.
This adventure is much more open in concept than I thought it would be, so I need to better prepare beforehand in order to be ready to improvise when called on to do so. That is fine, it's just important to know it needs to happen. I wasn't ready for that this week, but next week I will be. It is a fun setting, with tons of options, and it will be fun exploring them, bearing in mind that prepping will be needed.