Even leaving aside the bit about taking photos, checking instagram, calling into pubs and being misled by "helpful" way markers, the time it take to walk a particular distance can vary wildly, even for the same person, depending on the terrain. So much so that it was something of a joke on the Usenet group rec.arts.sf.composition that when someone asked, "How long will it take my characters to travel X miles from A to B?" the answer was usually, "How long do you want them to take?"
While we've been checking out the routes in G's geology book, we've had many instances where the route on the ground bears no relation to the enticing green dotted line on the map indicating a right of way. One path across a waste tip at a slate quarry had fallen away into the valley, several had disappeared into thick trees and tangled undergrowth and become impassable. Others had turned into linear bogs or just disappeared completely, like the path across open moorland where there were warning signs about not straying from the path due to hidden mine shafts. But what path?!
I can still do 3 miles in an hour on a good track, but some walks were more like 1 mile an hour or even less as we toiled up steep and rocky hillsides or wormed our way through almost impenetrable forest.