JOSM, in my opinion, is a bit over the top for an average route project. When I presented this approach a while back (
Open Street Map Vector Data in TransDEM with JOSM), I regarded this more as a feasibility study than as a standard workflow.
The easiest and most efficient way still is to apply Map Tiles. Follow tutorial #7 in the TransDEM
main manual ([F1] key in the TransDEM main window), page 146 in the current edition "Tutorial 7: Map Tile Services". It also explains how to use the built-in "Simple Route Editor".
Now, when you say, a particular map source may not be detailed enough and you want to switch to aerial images, there are a number of providers in the sample configurations, namely ESRI, Microsoft Bing and Google. (You will have to register an API key for Google) that might offer sufficient detail.
If you really prefer the slow path, you can indeed use Google Earth instead. You will need GE placemarks and it will be an image-by-image task. How to do that is explained in tutorial #2 in the TransDEM
main manual, page 130 "Google Earth™ and TransDEM".