I've posted some tips in another thread before:
viewtopic.php?f=21&t=15200#p269919Have a look there if you haven't already. In that post I had links to AAWT gpx track files, but haven't checked if they are still active. Have a look at those gpx files and and see if they might help you.
Also if you don't have maps for your eTrex 20 yet, have a look at andrewp's OSM maps with topo contours also linked in that post.
The very basics are in a few videos that Garmin made a while back
http://www8.garmin.com/learningcenter/o ... ail/etrex/, but you've probably seen those already.
While it is entirely possible to preplan a trip on the device itself, it is easier to use the PC software Basecamp (or the older Mapsource) to view tracks and routes before copying the route to the etrex 20.
When you're new to GPS receivers, it can seem daunting with the various options and settings. Even after using various models of GPS receivers, there are always new things I find that aren't documented in the manual.
I have the eTrex 30 and I'm in inner eastern burbs, and more than happy to help if you have specific questions.