When I lived in Texas we would go to Barnwell "Mountain". Barnwell Mountain Recreation Area
It doesn't have a lot of really long trails but it has a lot of trails all over the hill. It has camping, both primitive and luxury. There's a lot of trails from mild to moderate with a few serious hill climbs.
I never made it up there but there's some good trails in Clayton, OK: Wheeling In Clayton, Oklahoma - Jp Magazine