Stuck for an answer, I vaguely remembered that Oregon's Skyline Trail followed the backbone of the Cascade Mountains from the bluer than blue of Crater Lake to the sleeping white beauty of Mt. Hood.

That California had a Riding and Hiking trail that ran from the Mexican border and crossed the Mojave Desert and the Sierras. And that Washington's Cascade Crest Trail provided a final link leading to Canada. "Oh," I said casually, "how about riding from Mexico to Canada?"