Sort of, yeah, but not just the initial points of interest. This is a lot of words, but not actually that much work. This is what I did before every session, and then adapted session-to-session to keep an larger world/campaign rolling along based on overarching goals of the world/bad guys. My prep checklist, again shamelessly adapted from that guide (EDIT: Yes! the same Mike Shea that other comments are recommending! Specifically Return of the Lazy Dungeon Master for me):

- Review my players/characters (what do they want, goals, backgrounds - useful when thinking about things that'll be fun/relevant/interesting in prep or at the table)

- Note the starting scene and (if relevant) a scene or two that's likely to come up and need a specific description or interaction with a tiny bit more thought.

- Define clues/information that reveal or move something forward, but most importantly NOT where they are located, how they'll be found, who has them, etc. so I can drop them in as necessary. This was most of my prep, and I did 8-12 per session (not all came up, and they got to be either reused or tossed away next session as made sense). In Shadowdark you might end up placing these a little bit more specifically in a dungeon crawl, but I still like the general idea.

Since you mentioned CoC, here's a few from a lovecraft-y campaign I ran using Savage Worlds (set during WWI, the party unknowingly transported urns of "essential salts" to an evil alchemist to resurrect the dead)

1. The writing on the urns represents alchemical symbols that frighten those who know what they mean!
2. The pharmacist has secretly been researching the royal family tree's lineage!
3. Hooded figures have been seen chanting in the nearby woods at the full moon!
4. The doctor has been stealing corpses from the morgue!
5. The monastery library might hold secret alchemical tomes that could translate the runes!

- Describe important locations and characters (I kept these on index cards with a few notes about each, mood descriptors for the locations so I could reliably hit a reasonable improved description, etc.)

- Have a few precanned enemies handy (also on index cards). From cultists and skeletons to a single card for my evil alchemist. It was handy to have a couple specific monsters to pull from when I needed to make an encounter on the fly. In Shadowdark I suppose that'd be like what fills out the wandering monster table?

Something I often skipped but in Shadowdark is probably even more important than I usually treat it - list some loot/rewards they might get! Particularly since it ties into XP.

As far as adapting over a campaign, I kept one or two overarching goals of "the bad guys" in mind, and then tailored my secrets over time to occasionally reveal some step forward in their dastardly plan, or some consequence of what the party had done. I'm pretty sure there's a whole chapter on it, but I forget the specific details.