You found out the hard way that the maps Garmin sells on SD cards do not work on the PC. For that, you need the CD or DVD map products, all of which include the MapSource program to manage maps, routes, waypoints, and tracks; to display them on the computer; and to transfer them to the GPSR. This works best when the same map is in the PC and the GPSR. Then you can see the same roads, hills, points of interest, etc. I recommend the City Navigator for the part of the world that interests you for road riding and for the data you download from Adventure Cycling.
(For others reading this, consider buying the maps from Garmin on CD or DVD and a blank SD or micro-SD card from your favorite electronics discount store. Use your PC to copy the maps onto the card.)
Consider also the relevant Topo series map for back road riding, off-road routes, mountainous terrain, and the Great Divide Mountain Bike route. I keep both topo and road maps in my receiver, switching back and forth during a ride.