The fraction of the population that can sustain a 4 mph pace comfortably for three hours is probably small. That's a pretty good clip. 2.5 to 3 mph is much more realistic.
Yes. I've been walking for a LONG time and 12 miles takes me about 4 hours. That's a lot of time out of your day, plus you likely won't have a lot of energy left for errands or whatever else needs to be done afterwards.
I'm a pretty fast hiker most of the time, and I find that my pace almost always works out to 2 mph once I figure in breaks and miscellaneous stops or delays. Like fishing out sunglasses, taking bio breaks, removing or adding layers, or enjoying the view for a few minutes. (Granted we're not necessarily talking about hiking.)
The fraction of the population that can sustain a 4 mph pace comfortably for three hours is probably small. That's a pretty good clip. 2.5 to 3 mph is much more realistic.