Why can't directions be standardized? There might be less confusion in the world if there were. Cities confuse me enough without having slightly faulty directions to go on. So, here are my directions. I hope they make better sense.
From I-35, exit at Riverside Dr., just past the Townlake Bridge. Go west on Riverside Dr (the light), past Congress, to Barton Springs Rd. Turn left and follow it until you see the mass of humanity on the left, across from the soccer fields. Actually, you will see all the *vehicles* belonging to the mass of humanity. This is Zilker Park. I'm sure today wasn't an exception... beautiful weather... a Sunday... equals half the city turned out to enjoy the day. Which of course, you can't blame them for. I was there, too. :)
Good luck finding a place to park. It took me at least half an hour to find a place to park. At least in that time, I got familiar with the layout of the place. The trailhead is at the back of the park - you can't miss it, there's a big sign that marks it the "Trailhead". :)
It's challenging from the standpoint that you have to pay attention to the trail. The rocks will get you if the other trail users don't. Actually, I didn't see any conflicts, despite the extremely heavy use. The trail itself varied from very smooth hardpack to rocks you had to climb over, up and down. Most of it is rideable - it has to be for people to be out there with their kids. :) The trail follows the creek for the most part, and crosses it a couple of times. Is there ever water running in the creek? I thought there were supposed to be good swimming holes. But a week after a very heavy rain that overflowed our tank, Barton Creek was bone dry. In fact, it was so dry, and heavy traffic so apparent, the best lines and even the rocks were shiny!
I followed a couple of other people, because I figured bravery is in numbers. I had no idea where I was going, if it looped, went straight out, branched, or what. So I stuck close on the wheel of the woman in front of me, and had a pretty good, albiet crowded, ride. We went out... and when they turned around, so did I. There are several places where you can shoot off or join the trail - I have to admit that a lot of it looked the same. Especially when I was paying more attention to the trail than the scenery. :) I ended up with a 12 mile ride that didn't feel like half that.
Everyone using the trail seemed to get along really well, which was nice to see. The people were like a blur as we passed, but everyone seemed to be smiling and enjoying themselves and the day, and everyone did their best to yield the trail, no matter who was coming, wheeled, 2-legged... or 4. :)
If this trail were out in the middle of nowhere, I'd like it a whole lot better. But by the same token, if it were, half the city of Austin wouldn't have a place to bike on the beautiful sunny weekends in early February. :) And I can always find that elusive singletrack on the road less traveled... if I travel far enough. ;)
Update time, dear readers. Is parking a problem? Like to start your ride with a downhill, and finish with a climb? Then have I got a starting place for you! Turn onto Scottish Woods Trail from Loop 360 (coming from the north, it's a right turn at the traffic light just after you see the green sign for West Lake HS). Take that all the way to where it T's, and park along the curb on either side of the road, in front of the houses. On your bike, turn right at the T, and you will see the V-gate on your left. You either have to hoist your bike (like I do), or walk it through on its rear tire. You are at the top of the Hill o' Life. :)
There are concrete water bars, and some loose rock. My source says it's 3/4 of a mile long... probably the longest hill I've found anywhere around! The trail will level out into gravel singletrack, and you'll see the creek (which had water in it when I was there last) on your right. The entire creek isn't flowing, however. By the time you make your first crossing, it's dry. You'll take the first trail you see to your right, then merge left - another trail comes in from the right - it was closer to the creek, and on your right when you were on the gravel trail. You'll go straight across the rocky creek, and up the leaf-covered trail on the other side, to the left.
There are several branches in the trail... different places to cross the creek... Eventually, you should end up at Zilker Park. For restroom breaks, there is a public restroom in a building toward the end of the parking lot. I think there are also water fountains for Camelbak refills.
Then, turn around and head back to your car... at the top of the Hill o' Life. :)
There's a good mix of *stuff* on this trail; lots of smooth, twisty singletrack, the infamous Austin-rocks, creek crossings that will test your bike handling skills, and plenty of rock 'n root technical little humps to both climb and descend. For the most part, it's a flat run along the creek, with the only real hill of the ride, the Hill of Life at the end (or middle), depending on where you start your ride. I highly recommend this trail, especially to newcomers to Austin area riding.