There is also a good website for routeplaning on bicycles, which will help you. You can even see the gradients of the road and have different options (only bicycle paths, the fastest path, the shortest path ...). I tried it myself and find it more accurate most times than google maps for bicycles.
My suggestion would be: take only bicycle routes and try to avoid gradients. The RadroutenPlaner would suggest a route of 86,8km, having a gradient of 849m and (by traveling with 15km/h) you would need 5:47h.
If you have a GPS, you can even download the route you calculated.
Don't try to navigate to "Frankfurt Airport" ... didn't know that either, but Frankfurt has a district called "Flughafen" (which means airport), so it will navigate you in the wrong direction ;)
Try Flughafen Frankfurt-Hahn