I think what you would have to do is first, create the waypoints in the GPS and save them as 'favorites' so that you can then build the 'route' in your GPS. Once the waypoints are in there, you can create and save it as a route.
Although the GPS will essentially 'track' your travels... it doesn't exactly save that ride for any future purposes.
If you can remember the key points along the way, you should be able to pick them from the map view and then build the route to be saved.
At least that's how I do it for rides up here that I haven't completed more than twice.
For my trip to Texas this Summer, I simply used Microsoft's Streets/Maps/Trips (whatever it's called) planner to build the waypoints on my computer at 200 mile intervals and used the cities at those stops to plug into the GPS as Waypoints. Once everything was established in the GPS, I saved each day's ride as a Route. The 200 mile interval was for Gas Mileage only, as I plan to get anywhere between 500 and 700 miles in each day. Would like to take a little more time... but my leave balance doesn't want to participate.

Okay, as I often do... I went back and re-read your original post.
I've never tried to export a route from my GPS so that it can be viewed on my computer so all that I typed above is more than likely something you already know... sorry.
I've had the Microsoft program for so many years I can't even remember what it's actually called... but I start with it and then manually transfer the necessary information into my GPS... even though the software has the ability to transfer the information directly to the GPS. The software supports many GPS units, and I know that it supports Garmin, as I've actually used it in the laptop while traveling in the Tahoe, connected to another Garmin I own to physically show my position using the software... kind of nerdy I know, but it was just something to pass the time.
