For me, it would probably be the HH100. But a part of that is because it is only 3 hours away and I can drive up in the morning. If I lived in the Houston area, I might be looking closer to home.
The main attraction of the HH100 is not so much the ride as the overall event. If you had the same route, same time, and only had 100 riders every year, no one would get too excited about it. But the hoopla sort of feeds on itself. So it's worthwhile to me to go up there just so I can ride with 12,000 other people. Don't get to do that every day!
The HH100 is fairly flat, and the last three years that I've done it, it hasn't been terribly hot, either. Any rides in the Houston area itself will be even flatter (and the Space Race is the only one that comes to mind, but I'm not down there, either).
As far as fitness- get out and try some 100k rides in the summer and see how you're doing. If you can ride 100k and feel good, you're probably okay for 100 miles. If you do 100k and think you're going to collapse, maybe reconsider. Also keep in mind that Wichita Falls is drier than Houston, so effects of temperature aren't quite the same.
Check into Endurolytes or whatever it takes to keep you from getting cramps, which can be a ride-ender. Don't depend on the Powerade they give you to keep cramps at bay.