you need boats for rivers - how do you carry a boat when you travel from one river basin to anothe
you make one or a raft, plenty of material all around and with a few mates it is a question of day or two.
I don't take sides here, just saying.
guess what travel from say Wrocław to Gniezno would last equally long on horse as using rivers
It depends what tracks looked like those days but I would bet on rivers.Please stick to the topic