Google maps is a good place to start.

However, carefully examine the route it gives you and alter it.

I just rode a 260-mile round trip in which Google tried to route me through an off-limits military reservation. I caught that before I left and changed the route--but the new route it gave me was over a major two-lane 70mph truck route with no shoulder. I had to create another new route on the fly. Also, things like construction (and closed roads/MUPs with lengthy detours) are beyond google's ken. Google is a good place to START, it should never be the last word.

Another Google problem: it won't print usable bike maps. For that, I download USGS quads from Libre map project, shrink them down to 8x11 size, print them off, and mark my route. This is nice because it helps when re-routing on the fly as I had to do a few times on a 260 mile trip. The quads also have better contour lines than google.

I agree that what Adventure Cycling puts out are the best, but they don't often go where I want to go. If you, like me, want to go your own way, start with google. But don't trust it. It's far from perfect.