Delete all of the other tracks/waypoints etc in MapSource, then do File -> Save as, and you can save the individual track to a GPX file.
Or right click on the track, then copy, then do File -> New, and paste in the individual track. Then Save as to a file.
Both of these just crash Mapsource as soon as I Save As (with GPX file type)
Well that's not normal Mapsource behaviour, believe it or not.
If you haven't already, try updating (or possibly down-dating) your Mapsource to v6.13, just maybe that will cure it.
Easier than doing a full uninstall/reinstall.
File is on the Garmin site at
http://www8.garmin.com/software/MapSource_6137.exejust download it and run it, to do the upgrade. Copy your existing installed Mapsource.exe file to another directory first, for safety, because this update will simply replace that file.
NB this version number has nothing to do with whatever map version you have installed (v8 or whatever) - is is just the version for the 'wrapper' Mapsource program.