If you are into VERY challenging hikes I would highly recommend
Caprock Canyons State Park. It takes about five hours to get there from Dallas, less for you if you're already north of here.
It is great for tent camping if you go to the back of the park to the Little Red or South Prong tent camping areas. Notice the map states several "Extremely Steep & Rugged" warnings that are not to be taken lightly, even by seasoned hikers (I did--the first time). Making it to Fern Cave is quite an accomplishment and the scenic overlook by the Haynes Ridge Overlook Trail is incredible.
Greenstone Guide to Texas and the Hill Country: Caprock Canyons State Park
Here is a review of the park I found online. This guy has been to several state parks and has good informative write-ups on them.