I have a Garmin GPS, how can I extract a tracklog for feeding into Lightroom 4?
Most likely something like GPSBabel will work to download the tracks from the GPS into a .gpx file. There are many other programs that will work with the eTrex. Garmin provides free ones such as BaseCamp that will also work.
I use EasyGPS on my Windows 7 machine. Works like a charm with my Garmin Etrex Vista HCx.