About 30km/18 miles per day is a common value used in Europe. On a single day you can easily do twice that if you don't carry too much and are used to walking long distances, but it would be difficult to sustain. Also the number allows for time to set up camp (and tear it down in the morning), prepare meals, etc.
When I was a boy scout (so 16 -18 yo) we were walking as a team 50-70 km per day and could sustain that for 2-3 days. More perhaps, but there was nowhere to go any further. We were wearing 20 kg in rather uncomfortable backpacks.
On forced marches we could keep 7-8 km/h speed. And sometime we have practiced legionary walk which is alternating 200 steps jogging and 700 steps walking.
One of the badges was for walking 100 km in a day and lot of my friends got it.
But back then we were generally walking a lot, day by day, every year.
When I was in elementary school, we would do charity walks every year to raise money for the poor. They were 20 miles.
It took us all day, but if a bunch of kids between fourth and eighth grades and their nun teachers can do it, I'm surprised how many adults on this web site think it's too far.
Did you do it with a suitcase of stuff for a week- or month-long stay in another city, and contingencies like extra water, food, stove etc if you get laid over in a remote area without facilties? Did you do it rain or shine, on paved roads or muddy rutted-out dirt roads?
Well I did 30km with ~1500m height gained three-four times a week for a while. If I pushed I maybe could do 60km, but that would be no fun. Tops was 40 km on trails and 2500m gained, but that was extreme.
You need enough water, good shoes and nice weather for that. Or a flatter terrain.
On flats, 20km takes ~3.2 hours and yes you can walk 60 or 80 in a day and not drop dead at the end of it if you take care.
I wonder how long can a horse sustain that pace vs a human. Those who hike the PCT (Pacific Crest Trail), which takes 4-5 months, might do ~20 miles per day with a “zero” (rest) day every ~10 days.
The average PCT thruhike in 2012 was 152 days, or over 5 months (https://www.pcta.org/2013/how-long-does-a-pct-thru-hike-take...). Only skilled and experienced thruhikers are able to complete it, so there is serious selection bias in that figure as well.
It's only with advancements in backpacking and outdoors tech that anyone is able to do that.
I don't know why everyone in this thread seems to think titanium tent stakes and frame backpacks and gore-tex jackets and polarfleece and plastic pump-driven charcoal water filters and chocolate bars and first aid (that wouldn't kill you) were a thing during ancient rome.
