The answer to this question depends on many variables, like terrain (hilly/flat/boggy), season, the composition of the force (does the force include a baggage train? how many horses are avaiable?), the avaiability and condition of roads. Does that army expect an ambush, i.e. does it have to march so that it can give battle with little or no preparation? Is it a short forced march or an extended campaign etc?

The army of Harold Godwinson is said to have traveled from London to Yorkshire in 4 days in September 1066. This amounts to more than 40 miles per day, and was definitely very fast for that period. As the conditions were very good (mostly flat ground, march through friendly territory, no wounded, no extensive baggage train etc.), this may represent the high end of what an army of that period could accomplish.

The army of Richard Lionheart before the Battle of Arsuf was much slower. It started in Accre on August 22. 1191, and reached the site of the battle on September 7. The distance covered in these 16 days is about 60 miles. The crusaders had to march in a defensive order and had to make frequent stops near sources of water to prevent heat exhaustion. Also, they had to continually protect their baggage train.

The Schlieffenplan - the german plan for invading France in 1914 - estimated that soldiers would be able to march 20 km (12 - 13 miles per day), not counting the time spent fighting the enemy. A forced march was considered 32 km/day, or about 20 miles. This estimate assumed well-fed and equipped soldiers using the road network of 20th century france.

So, about 50 miles per day would be a resonable estimate for a forced march under optimal conditions for a short period of time. In most cases, armies would not move more than ten miles per day, and could move considerably less in adverse circumstances.