Detailed resources for planning backcountry/primitive tours? Are there any resources out there with details for specific backcountry/primitive tours, such as water sources, itineraries, camping spots, route difficulty, etc?

I'm a hiker/camper who just bought an "offroad touring" bike a few months ago, and I've been dying to take it out on a long-term trail ride. Problem is, so many of the trails that I find don't allow bikes at all. The ones that do are frequently very short and catered to technical mountain biking.

I've found a couple sparse sources, but not much. Most sources for tour info seem to cater to supported road tours, or road tours in general. This is the best example of what I'm looking for, but I get the feeling that there are many more options out there than this site lists. I notice that the entire Ozark trail is missing, for example.
Are there ever!

I'm totally evangelizing for the adventure cycling association in my answer here, but check this out: Adventure Cycling Route Network. They have a lot of road tours but they do mountain tours as well.

You want the extreme of extreme backcountry mountain bike tours? How does 4418km of trail riding along the continental divide sound? Check out the Tour Divide, It follows the ACA's Great Divide Route.

Other than that take a closer look at the trail systems in National Parks, there are  trails that permit cycling, and some that are long enough to bikepack on for a couple days. At least there are lots where I'm from, which is near Glacier National Park.