Fred has suggested to me separately that it might help other novices if I were to write a fuller account of what I did with the gpx files as downloaded from the ACA website. I have an elderly PC running Windows XP SP3 and I used Garmin's City Navigator North America NT maps on DVD in conjunction with MapSource 6.16.3, the final version of this software before Garmin switched its development efforts to BaseCamp. The maps came with BaseCamp on the DVD and I downloaded MapSource from the internet. I expect BaseCamp is capable of doing everything that MapSource can and more; I've only looked at it briefly. I used MapSource because that is what seems to be preferred on this forum. I find it prone to frequent crashing and with a cumbersome way of saving one's work.
Unzipping the ACA files and opening them in MapSource shows that each map section (= one paper map) is divided into a number of routes. As reported in my original post on this thread, there were 69 in all for my planned journey, omitting a few variant routings I knew I wasn't interested in. I thought that was too large a number for convenience. After some trial and error, I determined that one route per map section was a workable number. That would give me 11 routes in total which together would cover my entire journey (actually ten routes because I combined two of the map sections). Once on the road, it should be easy enough to scroll through ten routes to find the one I want on any particular day. So, following carefully the step-by-step instructions contained in the second of Fred's posts in this thread, I created new combined routes. For example, gpx file TA12v007, which covers map section 12 of the TransAm route, is made up of eight routes which I consolidated into one.
I also changed the names of the route waypoints, which are initially alphanumeric combinations like J0CAE0, into directions that would be meaningful to me on my bike. I derived these by study of the paper maps and occasional reference to Google Maps or Google Earth. I found a number of places where the GPS routing goes a slightly different way to the paper map. I reported these to the ACA using the online map correction form.
When I'd finished, I opened two instances of MapSource with adjacent routes; for example, I opened consolidated route 12 (from the eight routes in TA12v007) and consolidated route 11 (from the eight routes in TA11v007). I then added new route waypoints to the end of route 12 corresponding to the first half dozen or so in route 11 and new route waypoints to the beginning of route 11 corresponding to the final half dozen or so in route 12 (I'm cycling westwards from route 12 to route 11). The purpose of this was to provide some overlap between the two consolidated routes.
Having done all that, I transferred the completed routes to my Oregon 450 receiver using MapSource. I found it necessary to do this one route at a time. After transferring one route, I disconnected the device from my PC, restarted it and let it load the newly transferred route, then reconnected the device to the computer and transferred the next route. If I tried to do several at once, only the last one would transfer successfully. I had already transferred the relevant parts of the North America maps to the device using MapSource.
I hope this account will be helpful to some. I will try to answer any questions. If anyone wants to see how I make out on the road, I'll be blogging at http://aldernath2012.blogspot.com.au/