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.
Review: The Powerline downhill section is my favorite. There truly is something for everyone at this trail. Lots of trails mostly under trees. There is a BMX jump area by the East entrance of the trail.
Review: I got to the park 9 am on weekend to avoid the crowd. I love BMX loop so much fun. no hiker in this trail. It is hidden gem for sure. about 1.5 miles and you can redo it as many time you like.This is my second day doing MTB and I was be able to ride slowly through the trail. love it. Around 10am we heading to the main walnut trail. We have to watch out the hiker and rider. the deeper trail you in the better off because there is less hiker.
Review: Rollercoaster of a trail. Lots of variety for beginners, intermediate, and expert riders. Easy to get lost but never any worries. Even in summer, if you go in the morning, you get a ton of shade so not too hot. If there's rain recently, streams make crossing fun.
Review: This place is 5 minutes from my house and is pretty awesome. I've only been riding for a few weeks and I've been here the past two days. First time was a bit confusing, but the second day was better after taking the locals' advice and just following the arrows. I really like the log loop. The people are friendly, does get a bit more active on the weekends, but that helps if you get lost. I love this place, an oasis in the heart of Austin.