Adventure Cycling publishes paper maps and a few PDF files of maps that you can download and print. No GPS receiver can load these. If you want electronic maps on your handlebar, you need to buy a mapping GPSR (The Foretrex is not) and a set of maps to load into it.
Adventure Cycling also publishes GPS waypoints of significant points along its routes. You could use these, but not easily, as the Foretrex lacks a connection to your computer for loading waypoints.
In principle, you could read the name, latitude, and longitude of a published waypoint on your computer, mark a new waypoint in your Foretrex, and edit it to the published location using the Up, Down, and Enter buttons. Repeat for several more waypoints, then link them into a route. This would be terribly tedious; not at all practical.
Popular receivers to use with the ACA maps and waypoints are the Garmin 60C series and the higher end of their Etrex series.