This exactly is what made me wonder about the "immersion" of removing fast travel from Skyrim. I do this all the time in roleplaying because even if travel is dangerous, it's not necessary to roleplay out the entirety of it.
I tried the "do not fast travel" stuff in Skyrim for immersion. What it ultimately lead me to do was ignore most of the quests in the game... ignore most of the NPC's... and operate out of a "base town" where I stored junk. Wandering the same roads over and over again, again and again, for no reason other than "I need to get to where this thing is"... proved largely annoying. I did it so long that I knew when I was pretty much guaranteed a Dragon Encounter as they seemed to always happen along the same two roads at about the same locations. I knew where most of the "random" encounters were as well.
Not using the Fast Travel in Skyrim actually broke my immersion some by letting me get to know all the programming in it, because I was forced to experience it... again and again. The magic of "discovery" was stolen from me after about the 20th hour of doing it. It became tedious.
This sounds like a great alternative to fast travel. If I had to go back to a certain city but now I've unlocked quicker pathways that makes it a breeze to get there, it's in effect the same thing without stepping outside of the normal mode of transportation. (As for actually needing it in your game, that honestly depends on how much backtracking you have; see below)
It also serves to let you get AHEAD in the game as well. If you reach level 25 before going around that bridge... it still appears once you reach Level 25, which lets you into the next area.
There is a very large amount of backtracking in my game plus I needed to have Level Ups do something when I divorced stats from them. "Giving the player a reason to explore the map again" seemed like a great alternative. I then began simply expanding on it. More often than not, it opens up shortcuts. One of the earliest unlocks is at Level 4, where a bunch of freight is removed from the south entrance to the first town you run across. This allows you to bypass a large circular path around the map (passed two dungeons... an NPC... a forest... a long trek around about 250 steps) and simply get to the southern portion of the map quickly. At level 25, that bridge appears near that town, which gives you access to the eastern section of the map without having to go through a cave, a mountainous area, and a second town. In fact, the bridge effectively connects the two towns. It shortens the routes from hundreds of squares to maybe 50 or so.
These routes become largely necessary as you gather party members and come back to old areas, as the old areas now hold higher level/stat content for the new party members. The backtracking becomes necessary if you want to power up the new characters, and the shortcuts make it much easier to do (basically, I took the concept from Metroidvania type games where power unlocks quicker map traversal, and backtracking rewards you with more power... and the quicker map traversal makes that important).
Levels do more than just unlock routes though. But, largely, that's their purpose. Shortcuts across the map.
Backtracking is probably its own huge discussion, but after playing Octopath Traveler some, yeah it's definitely required when you're bouncing around the world like crazy. In fact, now I wonder what a Bethesda game that is linear would play out like without a need for fast travel.
It plays out... very... boring. Every encounter becomes predictable. Every trek becomes a chore. Once something is explored, there's no reason to do it again...
Fast Travel is great for large worlds where backtracking becomes necessary. Otherwise, you're stuck with the alternative of making your players bored. Making travel tedious.
