It depends on a number of factors. The first is the composition of the army. How large is it? Is it composed mostly of foot soldiers, or cavalry and mounted infantry? Does it have a large train of camp followers and baggage sufficient to support a prolonged campaign, or is it a stripped-down raiding force? Is it well-fed and in good condition, or is it hungry and sickly?

Second, is it moving over terrain familiar to it? Or is it a foreign army blundering around in relatively unfamiliar country? If the latter, is the population sympathetic to it? Does it have guides or access to maps showing roads and river crossings? Are bridges and other choke points likely to be fortified and defended, thus necessitating detours?

Third, what is the weather like? Is it intensely hot or cold? Are the roads muddy or washed out? Is forage available for animals (grass), or will it be necessary to pack the army's supplies?

Fourth, the least tangible, what is the morale and leadership situation within the army? Is it ably and skillfully commanded? Are the soldiers enthusiastic or dispirited? Are they veteran campaigners or at least trained warriors? Are they going to battle or fleeing from it?

Now, having outlined some of the factors, I realize I have yet to give you anything like an answer. I'm going to do my best to give an approximation. My caveat is that it's one that I can't specifically cite, because it's not drawn from any particular source.

Under best case circumstances, a smallish force of mounted warriors (knights, men-at-arms, hobilars and/or mounted archers, c. <5,000) might be able to do thirty-five miles a day for a period of a few days before wearing out their horses. If they're partially composed of foot marchers, that's going to drop to, probably, not more than 20 miles, and that's with minimal impediments and forced marching. For a vast army composed of the aforementioned semi-professionals, but supplemented with levies and other less trained troops, and with a large baggage train, probably not more than eight or ten miles in a day.

Sort of a tangential answer, but historically a loaded merchant caravan was expected to do a standard day of 20km. Thus Caravan Sarais on what we would refer to as the Silk Road (there were several actual routes) were spaced every 20km. It would seem like an army of any size with any sort of a baggage train wouldn't be faster but certainly could be slower, unless it was temporarily pushing hard for a specific objective.

Viking armies could move a 60+ miles per day by longboat on rivers and significantly outpaced land based armies (by 2-4x).