Actually, I made that exact trip in 20 minutes at the peak of the evacuation traffic jams -- by just staying away from the 290 freeway and using the surface streets...
And the gas stations away from the freeways had plenty of gas and no lines, too.
If we had to evacuate, I had an off-the-beaten-path route from Houston to Dallas (and points beyond) picked out in advance that would have gotten us out of town without having to approach within miles of any of the major evacuation routes.
What part of Houston are you in?
Congratulations. I watched it all on tv (satellite tv mind you) which never even went out once during the storm. I was amazed as well.