I'm pretty intimidated by the size and scope of these adventures myself, and I'm curious how much time other DMs have actually put into the prep for them. Did you read the whole adventure before you ran your first session? Did you create lots of auxiliary cheat and reference sheets? Pre-draw lots of your own maps, or buy and print out the digital maps off the internet? Make lots of photo-copies? Wing it at the table?
I've only played Princes of the Apocalypse, and that as a player at a game store for a few sessions. I know that the gentleman organizing those events put a ton of prep work into each session for the various DM's, but I think some of that was due to the constraints of D&D Expeditions.
I'm really intrigued by some of the ideas in this adventure, but just don't see how it would ever find more than a cameo at our table. Way too much investment of time, and I can't imagine sticking with one "adventure" for that long. I've never had a group get all the way through a 32 pp module without taking everything completely off the rails and discarding most of the final portions of the adventure.
I haven't prepped any of the 5e campaign-length adventures, but I've certainly done so in previous editions and for other games, so keep that in mind as you read the rest of what I write.
Prep-time for campaign length adventures should be focused, first and foremost, and IME you *have* to set some expectations with the players before you even bother. If the main expectation is that "we are playing this adventure path," than a little railroading here or there to get to the conclusion (or at least head in that direction) should be fine, and thus the book(s) should provide you with nearly everything you need, limiting the amount of prep you have to do (but certainly not negating any prep!). But if the railroad isn't interesting, and you're just mining it for ideas and themes, then you may have to do more prep (even if it's just to get some of the characters, plots, and places straight so you can strip them out of the adventure without undue page-flipping for fact-finding during play)...and that's where a lot of the little sideplots and stuff can come in handy, because that's what a sandbox is: a bunch of ideas that the players will truly make their own based on how they interact with them.
With that in mind, a really rough guide to prep is:
"Railroad": everyone buys into the campaign, mostly beginning to end.
1. Read the adventure once, thoroughly.
2. Pick the starting point, and the most interesting facts/info from the conclusion.
3. Create a quick-reference index of those things you chose, so you can find pertinent info at a glance.
4. Detailed read-through of the starting point you choose, plus that index of facts, so you can find as many possible ways to foreshadow as possible. After all, if you don't foreshadow (or if the adventure doesn't do it for you very well), then the players won't really know what the direction of the campaign is supposed to be, and thus could go off the rails pretty quickly.
5. Detailed cheat sheet of the starting point and maybe 1 or 2 connected areas.
6. Consider -- if you have a customizable GM screen -- creating screen inserts that are more tailored to the adventure, rather than generic rules references from the Core books. After a couple sessions of play, you'll likely master rules referencing, so that stuff is less and less necessary as time goes on. But knowing what page Sleezork the Naga is detailed on, and then what page his lair is described on...that stuffs important to have at a glance.
Don't go nuts making cheat sheets and indices for every stage of the adventure. An adventure like these takes MONTHS of play, so don't overload yourself right out the gate. Just detail what 2-3 sessions of material might hold, and you should be good.
FYI, I use "railroad" just for categorization; you don't have to run the thing like a true railroad. That's up to you and your group.
Non-railroad Method: the players like the theme of the campaign, but maybe don't necessarily want to care about the over-arching plot.
1. Follow the steps above, but don't worry about foreshadowing and indices of plot points so much.
2. Instead, make an index of locations, NPCs, loot, and plot points by locale (whatever seems logical), but only do 1-3 locales at a time (starting point plus 1 or 2 others).
This should focus your prep roughly by level; why do a bunch of reading and note-taking on the 15th-level encounters when the players are starting at 1st level? Worry less about plot, and just figure out what locations and NPCs are cool, maybe what treasures, and seed those amongst your players to see which ones bloom into points of interest for them. That'll give you the direction of the campaign, or at least the next few sessions, so you can save your prep for then.
Basically, if you like prep work, you can do expect to do a ton anytime you run something pre-published, unless you have photographic memory. But you can limit the amount of work by simply understanding where everything starts, and not going too far beyond that. If you enjoy the full on campaign ("railroad") idea, you may spend a little extra time thinking about the end-game, just so you know how to build to it properly, but other than that, you really shouldn't be going nuts prepping the whole thing ahead of time unless you've got loads of free time.
Also, the more you rely on just making shorthand indices of info, the less writing you duplicate. Adventure modules are rarely DM-friendly in layout -- there's too much textual info, not enough lists and bullet points, things are spread out all over the book -- but it's silly to rewrite the whole thing; just do page references. Use index cards (physical) or spreadsheets (digital) and sort your subject matter (NPC, location, treasure, organization) so that you can easily reference things already covered by the book. Both mediums (spreadsheets and index cards) have hard limits on how much text you can use on them without cluttering them up, so that forces you to think in shorthand and not overload yourself.
YMMV, of course, but considering the last two modules -- Into the Abyss and Princes of the Apocalypse -- were a little less linear than Tyranny of Dragons, hopefully this helps provide some focus.
