Well, I just found this:
http://orbis.stanford.edu/
It's an interactive map that shows precisely how much time it took to travel on roman times. it includes a bunch of different ways to travel (on foot, on horse, a marching army, on boats, with a cart, etc.), it also includes prices and alternative routes depending on whether you want to travel fast or cheap.
TL;DR: It's a roman empire travel planner.
And here's that doc in pdf format, just in case the Internet eats the original at a later date.
Thanks, AA!
Bryan Ray, visual effects artist
http://www.bryanray.name
What about while drawing a carriage?
If a convoy is traveling, and they have a pack horse and a sleeping/camper carriage, they'd travel much slower.