Nice cruise through the woods, mixture of single track and jeep trail. One loop probably will not be enough to wear you out. The ride accross the top of the falls is pretty cool.
From IH35 in Austin take 7th street east, follow the signs to 183 south towards lockhart. Take 183S to Scenic Loop Road which you will stay on til it dead ends at McKinney Falls Pkwy
http://www.singletracks.com/item.php?c=1&i=1322