I'd guess in the west through the mountains this is probably the easiest way to go and the sections I've ridden still have amazingly impressive scenes to enjoy. I'd imagine east of Missoula and through SD you'll have long gaps between rest areas, there were longs gaps just driving the route. The shorter sections I've ridden in ID, MT, SD, IA, IL, and PA weren't that much different from the roads I took to get too and from the trails and the towns along them were nice, I doubt the ride will be lacking by sticking largely to the trails except in leg pain avoiding some of the worst climbs.