The 6 Mile single track is a blast and has about 110 feet of climbing with enough short (but demanding) technical rock sections. Be ready to dismount few times on your first ride. It might seem short in lenght but it will take longer than you think; it gets more fun everytime you ride it. The loop runs one way which is clearly marked at the trailhead. The track was originary for Motorcycles that can still be ridden (be aware of them).
Take MoPac North to the RR2222 exit. Go left on Northland and then right on 2222 4.5 miles from the light at City Park Road. Turn left and take City Park Road 3.8 miles (staying right at the fork) to Oak Shore Road. Take a left on Oak Shore and continue .9 miles until you see the Emma Long Motocross sign on the left.
http://www.singletracks.com/item.php?c=1&i=1524