Idaho Backcountry Discovery Route. It's about 1200-1300 miles of mostly back roads and trails from Jarbridge, NV to the Canadian border. Idaho is only about 400 miles north-south, so there is a bit of meandering involved. The Lolo trail and Magruder corridor are part of it.
If you are interested in this sort of thing, ridebdr.com has a website with information about BDRs in several western states, but also NY, and Mid-Atlantic (Virginia to Pennsylvania). It's geared toward dual sport bikes, but most of the trails are doable in a capable 4X4. Some of the stages are just forest service roads and you could go with any vehicle, some stages are pretty serious and you need serious equipment.