Here in MN the state puts out PRIMS, that are sort of "county" maps. They have all the little farm roads, and culverts as well as some of the bike corridors. I didn't know there was a legthy route from Mankato to Faribault, virtually all rails to trails. Pretty sharp, and looking forward to including it in my southern MN tour.
Check with the DNR of the States you'll traveling to see if they have anything along these lines.