Hey Ed, about where in South Texas?
If you are close to the Gulf, you can run in most of the beaches.
Also there is an area near the town of Victoria called Coleto Creek where it's supposed to have a lot of good forest trails, river crossings and mud.
There are trilas also in/near Houston, San Antonio and Austin.
If you let me know around where, I'll try and ask around a bit.
Good Luck
Javier
Dallas, TX