welcome to the TON.
actually, the Thorndale place was an AMSA (Austin MotoSport Association) membership Lease from a few years back. it is now being turned into a high fence exotic game ranch and is no longer a AMSA lease.
CTOR (this forum) is located outside Salado to the west of IH35 and is a member only riding lease. I personally haven't ridden this piece of property, but used to ride several miles to the west when Fort Hood was open range riding and should be similar terrain. probably mostly calache rolling hills with cedars.
another option to the Austin area is another previous AMSA lease that is now an offroad park called Cherokee Cycle Park located a few miles south of Milano that has alot of trails catering to dirt bikes & quads. camping area and I think they have recently installed some RV hookups. various types of riding, some sand, hard pack, not too many rocks that I recall.
another option is Hidden Falls (formerly Boulder Creek) which is about 7 miles to the east side of Marble Falls on FM1431. they have trails for dirt bikes, quads and also full size 4x4.
another is Cross Creek Cycle Park about 2 miles to the north of Paige that has several tracks (MX, SX, kids, GP/old school MX) and some trails in the back section. I believe the trails are closed off right now since cattle were moved in for Ag reasons.
then there is Austin Del Valle MX just east of the Austin airport a few miles but mainly a MX facility. they used to have some trails to the west side but not sure if those are still there or maintained.
all these I have listed have forums here on the TON and one of the first threads under each should have a link to their websites.
I would also recommend you joining up with the AMSA as they are a great group of off-road (and some on-road) motorcycle enthusiasts, they plan a family ride at a local private property almost monthly, member discounts at a number of area motorcycle shops (and various other shops)...
~':/ Marcus \:'~
Laughter is the best medicine!!!
(I only have to amuse myself)