Depends on a lot of different factors.
It could take you 4 days to go 25 miles in the mountains, but it could also take you 1 day to go 25 miles in the same mountains.
The big factors to consider are:
The terrain under foot:
A flat boulder field is going to take longer to cross than a ridge of steep grassy hills
The elevation:
Mountain air has less oxygen. That means more effort to go the same distance. More rest stops, less forgiving if you make a mistake
The weather:
Heat means you have to stop for water more often or carry a heavier load of water. Precipitation affects terrain. Rivers may be swollen and take longer to safely cross or you may have to divert around them. Landsides (caused by the weather) may mean the path has been taken out and you have to divert around or scramble across an unstable scree slope. Extreme conditions take a heavier toll on the body. Blisters from soggy boots or chafing bag straps cause delays when taking a break to fix, slow the general walking pace down
How well you know the path:
It's not just about how long you spend deciding which branch to take. Often, it just isn't obvious that the path has split because one branch is hidden by vegitation and/or the lay of the land. I'd say not having a guide or local, even if you have very good maps, will probably slow you down more than anything else in this list
The population density:
You may get held up by the farmer herding his cattle, avoiding an ambush laid by bandits or locals inviting you in for tea. Equally, you can save time not pitching tents / finding shelter if there are lots of villages to stay at, and also save by carrying less food (as you can just buy as you go along). Equally again, you may find there are lots of villages nearby then no villages for quite a while. That may affect how far you are willing to walk. You may not be able to do the 10miles to the next village having already walked 20 miles that day, so you stop in the village where you're at. However, you've still walked less than 25 miles that day
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Anyway, this is why a local would say "Holtingford is a week's walk away" not "Holtingford is 100 miles away." There is a reason country miles are so inconsistent