Be careful after rains the trail gets slick. Lots of cover by trees and plenty of fun jumps. Watch out for runners and dogs.
Added by harryt: The trails in Walnut Creek park seem to be designed for you to create your own workout. I haven't mapped the entire crisscrossed system I imagine there is 15 miles of of sweet singletrack with some doubletrack and one jeep road.
There is little sustained elevation gain, but when trying to create a challenging work out I've gained over 2400 ft. Trailhead for the main system of trails is near the pool parking lot. Cross the creek for more challenging sections. Especially explore the southeast for a series of trails up and down the same ridge. Don't miss two isolated sections of trail near the park entrance from Lamar Blvd. both to the right and left. The trail to the right as you enter the park has several BMX style jumps (wear a helmet!).
I hope you have fun on this gem tucked into Northwest Austin!
Update 7/26/2014: Log Loop is going to be closed for a while, (month or so) while they construct some bridges as part of the park renovation. You can still hit Log Loop if you go opposite direction from normal blue path via Retention Dam.
MoPac north to Parmer Lane stay on Parmer til Lamar, take a right, entrance to the park is on your right.
http://www.singletracks.com/item.php?c=1&i=1327