Interstate is tough. But for long trails, there are some in MN.

Some specific ones are: The Katy Trail. The Mickelson Trail in SD, which is very scenic but not flat. The Olympian Trail in MT, which is less developed, not flat, but scenic. It connects with the Route of the Hiawatha Trail, which takes you into ID via St. Paul Pass. You can also transfer from the Olympian to the NorPac Trail that will take you across Lookout Pass into ID where you can connect with the Trail of the Couer d'Alene. The NorPac is also more rugged than your typical rail-trail and not flat, but also very pretty. It's technically open to vehicles since and is managed by the U.S.F.S. I saw one car when it rode the MT side. What little traffic there is moves very slowly. The Cd'A trail is paved. And let's not forget the GAP/C&O combination that will take you from Pittsburgh to D.C. In PA there is the D&L system, which has a few gaps in places.