Where is the best place in Austin Texas to do a real deep woods camping (camping not prefabricated)?

I am looking for a natural where you can go hiking in the woods or the mountains a few miles and just set up camp in the middle of nowhere. I know that most national and State parks have regulations on camping wherever you choose. I want something rustic, away from families, the most isolated, the better. Im looking for something within 75 miles of Austin, Texas, but I'm open to all ideas.

Remote Camping National Forest is available to find one near you here http://www.fs.fed.us/recreation/map/state_list.shtml

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