Could've sworn I posted this yesterday.... but....
Don't know what part of Houston you're in--sometimes it takes 3 hours just to get from one end of that city to the other!

That said...
...don't get scared away from trying some of the great hill counyrt SP's about 3 hours or so from Houston (Katy, at least) I have never been shut out of a SP, even though they say they are sold out on the websites. There are always cancellations, you know. Just ask the rangers what time to check into the park if there are no-shows. I have done this several times at Guadalupe River SP. My theory is that a lot of SA locals reserve spots, then cancel because of last minute plans changes, and it isn't that hard for them (us) to hit the park the next week or so (I've been in that spot, too....)
have fun.