I have gone some good distances on the Main River Bikepath. I liked it very much. I have detailed vivid descriptions of it in my manuscript from a 4500 mile tour I did in 1994 through France, Germany, Czech, Poland, Ukraine, Moldav, Romania, Bulgaria, Greece, Italy, and the east coast of the United States. It goes through sunny fields and trees and small beer gardens, immaculately trimmed lawns, and is right next to the river in many places. Many younger people were out cycling with loaded touring bikes going from hostel to hostel. If the Frankfurt hostel is still in the same place, it is a short distance from the hauptbahnhof and is on or very near the Main. Call them. Maybe they can take bikes.