Someone can walk about 40 - 50 miles per day flat out but thats putting in a really good pace and they would have to be used to it. A more normal person would do about 20 miles over a hard track and probably about the ole D&D 12 miles on a rough track or over hard natural terrain. That assumes good weather and good ground. As soon as either change it can drop dramatically. Over a bog I would think even 5 miles would be good going.
Also, sticking with roads, if a person was on a mission or quest then it could be up to more like the 20 mile figure and they may rest less. If doing a messaging relay then the ancient marathon would have been done in about 3 hrs which is something like 10 miles per hour so theoretically with multiple trained runners you could do 100 miles per day. Each one has no rest during the run.
Then if they were burdened with a pack then it would drop again and in armor like plate mail I wonder how they might move at all but maybe they could do 5 miles. An Elf or a Dwarf may do more due to fantastic racial traits.
I think we have had some more posts about this but its been a while since I have seen them. But do search.