These are modern people that sit behind desks all day and occasionally do this. The Tarahumara which train all of their lives for running can do 200 miles in 2 days, for example. The Inca moved fresh fish from the ocean to Macho Picchu in a few hours. See also Apache Spirit Runners.Sorry, not to nitpick... but ok I'm totally nitpicking... the bolded part above is way off. There is nothing easy about it.
Here are the results of a popular 10 mile road race. Only 27 out of 946 people ran faster than 10 miles per hour.
Here are the results of a 20 mile road race known as a Boston Marathon prep race. Only 6 out of 582 participants ran faster than 10 miles per hour.
These people are avid, gifted runners who train a lot. No one is JOGGING at 10mph for hours. Certainly not adventurers carrying gear who don't train at distances (at least I haven't had any of my players ever say "my character goes out for a quick 10 mile run" during most every rest or downtime opportunity).
Point being, short bursts of speed for under a minute are far far more plausible than maintaining that speed "easily for a few hours".
EDIT: in case anyone is looking for the RAW rather than some real world analogy, the PHB says characters can travel 4 miles in 1 hour at a fast pace... (p 181)
I never made the claim that any ole person can jog at a 10 mph for hours, but most adventurers are much much much more active than modern humans. When I was in track, people that could not maintain a 10 mph pace for a couple of hours did not make the long distance running team.
Also, most people that show up to run marathons are not in that good of shape and don't train for them. Having run quite a few I can easily attest to this. A 10 mph pace is a 6 minute mile. Most humans cannot run a 10 minute mile. However, in the time frames we are talking about for most combats, 4x and 5x running speeds for adventures are not out of line. I don't think I have ever played a D&D combat that took 10 minutes of game time.