This is a must do. We Took bikes on the bus from Fussen to Reutte in Austria. After touring Ehrenberg Castle ruins we rode home to Germany. The path runs beside the river Lech, with only a couple easily manageable hills (I did it on a townie). This path traces the route of the old Roman road that ran up to Germany to Italy. You could ride both directions, but I think we had more downhill. Since there is a ton of hiking at the castles I would say this is the best way. The bus out allows an early start before the crowds.