Read Thread: Boxes on a route?
Go to the Trip Planner under the Toolbox pulldown menu. There are dozens of pre-defined routes available, mostly along major roads. If you want to create your own you can. I just recently created a trip from Arlington, Tx to Newport, RI for a road trip this summer. After inputting the start and end points and a trip name you can add features and adjust the route on a map that comes up of your route. After it looks the way you want save the route. AQ chews on it for a while and then you have a route! After that you can search for letterboxes along the route (within 5 miles of the route is the default). I take the search results and download them to my GPS as a .gpx file but that is a whole 'nother tutorial.