This is an awful problem. It is one of those things that I have never been able to find on the net. The best way to do it is to find a historical record for a trip over a similar type of terrain, divide by the number of days to get an "average" number.
So a really long term travel without support over a variety of terrain... we could use Lewis and Clark's 8,000 miles in 28 months. Which approximates to 10 miles per day.
Oregon Trail was 2,000 miles and made in an average of five months, using cart and oxen, covered 13-15 miles average per day. They knew the trail.
Legionaries, professional travellers on a road with supplies, can make 30 miles a day. But if they are out of civilized areas, they make about 10. Of course SOP was to travel 10 miles, cut down some wood for a palisade, make a fort, and camp.
Of course, you could do it in relays, like the Pony Express. The Pony Express advertised that they would deliver the mail in 10 days or less. The trip took 1,840 miles. Each rider would go about 75 miles per day, with fresh horses approximately every 15 miles. The mail pouch would travel an average of 200 miles per day. The Pony express shows you the extremes you can do.
Now more serious travellers, who is crossing a given cross country might make 20 miles a day, average. They might push it to 30 per day, if they were good and there was a road and ready supplies. Of course, you can break this down, for the distance traveled over a given terrain type... but you get the idea.