Hi Tina
I'm not from the S Highlands, but know some trails there. If you haven't yet found them, the Mountain Bike trails in Wingello state forest are AWESOME! (And I mean for running, as well as mountain biking!). There are 3 routes, all off road, varying distances and technical levels, all very clearly marked. I ran the 25km route last weekend and had a ball! The trail heads are all at the same place, so depending on your level of training you can do one or two of them back to back to get in more kms. Visit
http://www.wildhorizons.com.au/highlands-trails for more info. Have fun!