It took me 72 days (three days of rest included IIRC) to cover the 1000 miles from Le Puy to Santiago de Compostela carrying about 10-15 kg. I bought food every day and slept in pilgrim shelters. Roads were good and straight and the few times we travelled on old roman roads pace slowed down. Rain also had a very strong effect on how fatigued you were at the end of the day. 8 hours in good weather or 9 hours marching in constant rain has tremedous effects on your morale
We travelled about 15 miles a day. I would imagine an army would travel about 5-10 miles a day. Break camp etc takes time. Carrying supplies is a chore. You get the picture.
There is no consistency of scales in Dominions. To build or besiege a fortress in months is silly. A population of a couple of thousand inhabitants in a province that takes a month to cross is also silly. An army of 300 units is also silly. We wanted to incorporate many features in the game. This is not possible unless you break some conventions. The current compromise tries to give some semblance of realism, but it is more intended to make for an interesting game than an overly realistic simulation of mythical warfare
