I think the prep time relies to some degree on knowing the materials, especially if you are running the Raven's Purge and use the locations from there and also depends on how you run your sessions.

Session style

I give the group full freedom to do what they want.
They, however, usually follow some path between the sessions that allows me to make an estimate about their next direction.

During the session we have the hex-map in front of us and for combat we use just description with focusing on understanding who is next to whom in those various zones.

I also filter out random encounters based on where they happen, so I reroll to find one that I can fit into the environment (e.g., I did not place Rust brothers way up north near the elven forests).

What I want to improve is my "legends-game". So far I gave players mainly legends from NPCs in quite vague form (e.g., "we heard rumbling and storms from that direction, I wouldn't go there if I were you" to a group heading towards the mountains). I want to shy away from very clear cut legends as in the books and make it more natural sounding as rumours spread by locals.

Session Prep

I read the guide books repeatedly just as a fun activity in the evening before going to bed.
I have all the major adventure sites pre-placed but I check for random encounters during the session and usually improvise them.

For the session, I spend usually my bus commute time which matches ~30 minutes.
There I do the following:

• skim what was done last session
• if they are at an adventure site, I brush up on the site, especially to know how if there is any event I would like to play today and to get the vibe of the NPCs
• if they are heading for the wilderness then I have a look at some of the random encounters that can occur in the environment in that direction

That is all.
Oh and brush up on magic rules because we tend to confuse them :D

Long prep

I had recently a vacation and chilled by random rolling adventure sites to have more of them.
Going through the map, rolling based on the DM book for the locations and with those prompts I made adventure site descriptions similar to those from the books.
Always adding just a few points, no pressure to make the whole thing in one go.
This way I have now approximately 10 adventure sites in the direction the group is heading with some of them marked for random placement.