I am looking for a nature area where we can hike into the forest or the hill country a couple miles and just set up camp in the middle of nowhere. I know most national/state parks have regulations regarding camping wherever you choose. I'd like something rustic, away from families, the more isolated the better. Im looking for something within 75 miles of Austin Texas but am open to all ideas.

there is not a place like that with in 100 miles of Austin. You will have to travel to east Texas in the Piney woods and even then there is not a "middle of nowhere", because they have designated camp sites, at least during hunting season, then there is always Big Bend, but i have never camped in a national park.