A tweet from Jim Phelps put me on to the Gmap Pedometer, which I’ve been having a quick play with this evening.
The service provides a means of being able to pre-record a route that can be exported to a gpx route in the same way I’ve shown previously. Rather than use Google’s directions service to be able to describe a route, you can simply ‘draw’ it on the Gmap Pedometer service and then export that out for use on your Garmin as before. Now that’s more like it – as provided the high-res imagery is available in satellite view, you should be able to record off-road routes without too much trouble.
A word of caution though. The registration/positional accuracy of the Google data coupled with the accuracy of your GPS unit at any given time may well lead to some pretty large error in positioning. Pretty large meaning, don’t plot any knife-edge routes and expect to be able to follow them blindfolded wihtout mishap. You have been warned!
Related posts:
Paul lives, rides and works in Cardiff, South Wales. His work seems to largely entail fixing things and keeping wheels on. Officially, he is actively engaged and interested in Enterprise Architecture these days, but has a secret past that involved standing around on glaciers collecting meltwater samples, and walking through endless fields of wheat taking radiance measurements. 
I should have read this first
before commenting upon your later post.
However, it would still be useful for an “instruction manual” – once you’ve refined the “system” / “process”.