If you are planning on playing through the entire game, you're looking at - realistically - 200 or so hours of gameplay. If you bite off a little at a time throughout your gameplay, this achievement will eventually pop for you.
Once you have a stable, buy the swimming ability, and then any time you need to cross a river or body of water, do so on horseback. Those little bits travelled eventually add up.
I accomplished this task after about 150 hours of gameplay. I had not yet started Francia, and I just finished up Eurvicscire questlines.