I don't recall if this has been discussed here before, but I found this interesting. My son sent me the link this afternoon. http://orbis.stanford.edu/
I missed this thread initially but stumbled upon it today..... this is quite an amazing tool. You can set the time of year to travel, the type of travel (by sea, by land, on foot, horse, oxcart, etc) .. So for instance travel from Rome to Constantinople would take 39 days by horse in the summer, 73 days on foot or 19 days by sea.