A tributary of the Main River runs northwest of the center of Frankfurt. There are paths on both sides of the narrow river, heading 10.5 km from Höchst north to Eschersheim. This makes for a nice run just outside the city, and the area starts to feel like countryside quite quickly. North of the major road intersection in Rödelheim, there is lovely farmland to look at as well.
The full route is very flat 10.5 km, one-way. At the southern end, the Frankfurt-Nied S-Bahn station is only 6 minutes from central Frankfurt. At the northern end in Eschersheim, there is an S-6 station at Frankfurt-Eschersheim or the Weißer Stein U-Bahn station. Key segment distances, heading SOUTH from Eschersheim:
2.5 km Praunheim
4.2 km Rd. 66 in Hausen
5.6 km Solmspark
7.4 km Rd 5 & 648 interchange near water park
10.5 km Frankfurt Nied S-Bahn