As I've whined about in the "Prisoners of the Blight" thread, the details of how traveling to the Blighted Depths work are incredibly vague. There's nothing on the exact distances involved, how quickly the party can travel them, or how often they will be exposed to Darkblight along the way. This has been frustrating to me, and I'm sending my players into the thick of it next weekend. Disclaimer: running second edition. So after 8 months of contemplating it, here's my current plan:
The book says the largest pocket of Darkblight is about 50 miles across. If the Blighted Depths are at the center (not specified, but a safe assumption) then the party is looking at about 25 miles of travel. I was actually assuming something similar to your travel guide and calling it greater difficult terrain, so with the party's 30 foot speed (might be 35) they are looking at about 3 days of travel. One could absolutely justify it taking even longer, but 3 days in highly dangerous territory is already a lot, especially since leaving will be difficult.
The book lists a small number of events during the travel:
A) Battle with Kusana at the Runestone border, performing runestone ritual to gain compass.
B) Finding destroyed Quickling Village, and beginning to be tailed by the potential quickling ally Wendel. Also an opportunity to recognzie the tracks of the bandersnatch-- I was thinking I might only let my players with Survey Wildlife make that check.
C) Battle with a giant sundew and a bunch of will-o-wisps in a weirdly non-blighted glade.
D) Arrival in the Blighted Depths. This is the first grid map the book has, being essentially an outdoor dungeon crawl with multiple encounters. From this point on there's little to no overland travel, and it isn't long before the party fights Naphexi and gains a ring which effectively renders the lot of them immune to the darkblight. The Runestone compass fizzles out at this point, but the terrain isn't constantly shifting here either.
In addition, rather than use the clunky "30% chance of random encounter every hour" model suggested in the back of the book, I'm leaning towards just scheduling the most interesting random encounters in the back of the book. I was thinking the two inluence encounters and fighting a bunch of elite leukodaemons, whose Quicken Pestilence ability would be help make the Darkblight disease a much more urgent concern. So a timeline would probaly look like:
Day 1: Hag encounter, entering Darkblight, Green Dragon Influence Encounter (I figure she'd be on the edge of the blighted territories, as this would be safer for her and let her act as a gatekeeper.)
Day 2: Quickling Village, Fey Reclaimer influence encounter.
Day 3: Sundew and Will-o-Wisps, leukodameons
Arriving in the Blighted Depths would be done either late day 3 or early day 4, depending on what pace the party keeps. I could add another encounter or two, but I'm not sure feeling especially inspired. There isn't much left in the PF2 bestiary worth meeting. I might add an encounter for this witchcrow from way back in book 1 who has been unwillingly transformed into a phoenix.
I think Planar tracking might make it easier to backtrack out of the Blighted Depths though... The compass probably provides no help in getting out, especially if it fizzles out when it actually reaches the Blighted Depths. So that would probably be a DC 40 check, but being able to follow an interdimensional trail of breadcrumbs back out would probably only be DC 30, maybe with an increasing DC based on how long the party spends in there and how cold the trail gets