Basecamp has a tough learning curve and it's pretty counter-intuitive. I liken it to learning Adobe Photoshop back in the old days.
Learn to create routes using waypoints or shaping points, and be sure your activity profiles match in Basecamp and on your GPS (Fastest, Shortest etc). Also zoom in when placing a waypoint or shaping point; Basecamp really cares what lane you put it in. For example, if you're planning a route eastward on a divided highway and you accidentally put a point in the westbound lane Basecamp (and the GPS) will want you turn around to go back and hit that point, then turn around go to east. After all, that's what you told it you wanted.
Rather than doing one long route, you may need to break it into more manageable chunks especially if you prefer to avoid interestates except bypasses around cities.
