Once you've hit singletrack, just follow the mountain in a loop near the base. On the northwest side of the mountain you can either keep riding up the hill or drop down onto Dripping Springs Road, then get back on the trail to ride back towards Geothermal or head West on Dripping Springs Road. This turns into University, a quick hop skip and jump from the Pan Am Center.
The best way to get to this trail is to park at the Pan Am Center, then ride south on the Pan American Highway until you hit Geothermal Dr. Take Geothermal East, it will turn into a dirt road. Keep following it towards A mountain. Right near the base of the mountain you will see a jeep trail crossing the main road. Jump on this and it will take you into the single tracks.
http://www.singletracks.com/item.php?c=1&i=1849