There is no single formula to help you out here. Weather, obstacles and other conditions will vary so widely you can't make any sort of predictions reliably. The best you can do is to posit ideal conditions and maximum range, so you don't say something silly like "they walked a hundred miles that day and arrived in late afternoon."
Check out ORBIS: The Stanford Geospatial Network Model of the Roman World which is interactive and really neat. Now, Rome obviously had roads, and the Mediterranean Sea was a big time-saver that connected a great deal of the empire, but it gives you some ideas--especially if your world has decent roads.
Six months to cross the Roman Empire end to end on foot. The program appears to assume a consistent 30 kilometer a day pace. Or spend a month and a half at sea.
"Average human foot speed over broken ground is 4 miles an hour."--Tommy Lee Jones's character in "The Fugitive"
Useful -- except, how broken is broken? We need to know that enough to describe it.