just try to have someone hiking or running with ya.. lately some of the trails are a bit "rapey or stabby".. but if you have a partner with you, not so much.
Balcones park in N. Austin. About one mile into the trail is an ethereal spring adjacent to the stream under an oak canopy. Natural air conditioning on a hot day. Magical place.
Thanks to Victoria for mentioning it.
I just ordered a book called Best Hikes near Austin and San Antonio. Has some trails in it i didn't even know about. May want to buy one if you like trails and hikes.
My recommendation is not a biking trail, but if you want the best hiking trail in Austin to help you get in shape (challenging), the River Place Nature Trail is my first choice.
Concur on River Place being a great place to hike, and pretty safe. Bring a camera as there are some great spots for photos. If you bird watch it is a great place for that as well; there is a Great Horned Owl family resident and you can hear the deep hoot at times.