I wanted to follow up on my popular Rhythm of Procedure post with an example of a deduction one can draw from it (and that I think one should) about rationalising the pace of overland travel.
The concise version of it is that one ought to structure the pace of travel so that the PCs travel either 0 or a whole number of hexes per iteration of the overland travel procedure, from the centre of one hex to the centre of another.
This probably seems obvious, and yet you get versions of overland travel where the hexes are 6 miles across, overland travel moves you 20 miles per day, and going over sufficiently rough terrain reduces your speed to 25% of its normal rate, so you might move three-and-change hexes, or you might move less than one, depending on terrain.
These fractional moves need to be tallied and accounted for over time (every three days of travel at full speed you will get a bonus hex of travel) and break the self-containment of the procedure's iteration. Moving back to a system where the vast majority of movement is whole numbers of hexes (with the possibility of an occasional failure to progress at all) is a way of restoring that self-containment. I think AD&D 1e did this but don't care to check or use it as a precedent, but I mention it in case you wanted to see such a system.
I think most of the OSR has arrived at a consensus around 6 mile / 10 km hexes being ideal for overland travel, primarily based on this decade-old (!) post from the Hydra's Grotto that makes the case persuasively. I favour "watches" (the basic unit of time of overland travel) lasting about 6 hours (Tho' I didn't get the idea from him, Justin Alexander has been calling these "watches" since at least 2012. He uses a 4-hour watch instead of my 6-hour one).
An unencumbered person without impairment walks a kilometre in level, clear terrain (like a city street) in about 15-20 minutes. So in six hours, we could expect them to walk 18-24 kilometres under those conditions, albeit they would probably be pretty tired by the end of it. But PCs travelling overland are usually not traveling in level, clear terrain; they are eating, defecating, resting, stopping to orient themselves, etc. sporadically throughout their travels; they are finding the easiest path through a hex and are avoiding calling undue attention to themselves; and they are travelling slowly enough to be ready to fight monsters, avoid ambushes, or perform other daring deeds during or immediately after the end of their travels. So I think it's reasonable to cut their travel pace nearly in half, and make it 10 km per 6-hour watch of travel (or 6 miles if you use American units).
Because this is such a leisurely pace, I wouldn't cut it due to PC encumbrance in most cases. Similarly, it already accommodates the slowing effects of most sorts of "rough terrain" (forests, wet meadows, ridges, etc.). Taking 6 hours to go 10 kilometres is a slow pace for a troupe of boy scouts. so rather than fiddling around "Oh, now you're in forest and you're going half speed", just accept they're always going about half-speed unless they have a reason not to be, and that half-speed is 1 10km hex per 6 hour watch.
There are three unusual cases that I think could use spot rules here. One is when you're on a road, one is when you're rushing and trying to pick up the pace, and the last is nigh-impassable terrain.
Roads and paths in normal terrain allow you to add one hex to the distance you travel in a single iteration of the travel procedure.
Rushing means the PCs conduct a fast march. They don't try to avoid people noticing their passage; they select the most direct route instead of the easiest; they don't stop to eat, crap, or rest; and they are probably going to arrive at their destination exhausted.
The rules here are pretty simple:
The PCs move 2 hexes instead of 1 in a watch where they rush.
They each make a saving throw at the end of the watch, and if they fail, they are exhausted and must rest (iterate the rest procedure) before marching again.
Riding horses or riding in wagons / carriages means the mounts make the roll instead of you.
If no PCs or mounts are exhausted, they can continue rushing. If some are, they can be left behind if the others want to continue rushing.
Nigh-impassable terrain covers your bogs, swamps, mountains, and other types of terrain where the problem isn't so much that they slow down your pace of marching, but that they are impassable without either specialised gear (e.g. mountain climbing equipment, boats) or specific paths (valleys, passes, solid ground, etc.) What counts as impassable varies based on whether you are riding, boating, or marching on foot.
Nigh-impassable terrain is impassable until a path is found. PCs must search for a path through the hex (taking a watch) unless they already know about one. Once they have found a path, they can then travel through the hex on their next watch. Depending on terrain, paths have concealment scores of >6.
The search takes 6 hours once again assuming the PCs are moving slowly and carefully, and that they must explore a number of dead-ends, false-starts, and the like to find a proper path. The right equipment in this case would aid the search roll since it allows one to convert more marginally traversable paths into usable ones.
e.g. If you have the right mountain climbing gear, you could choose to move between two ridges of different elevations that would allow you to traverse the mountain range in a way that you couldn't exploit if you couldn't actually climb from one ridge to another.
These rules are just proposals tho'. The core conceit is that regardless of the rules you use to implement it, the pace of the travel procedure should move PCs a whole number of hexes in each iteration for simplicity's sake, regardless of terrain or other obstacles.