some quality responses here, nice. Here are some thoughts I have:
1) plan your route to include refill opportunities: parks, gas stations, etc. Heck I stopped into a city rec center a couple weeks back. When it's hot, gotta have water
2) summer + south texas == major pain. DEFINITELY look into electrolyte supplements in your water. When I did the HHH a few years ago, pickle juice was a rest-stop favorite for a lot of folks (not for me). I use Hammer Endurolytes, but the NUUN or other products work too. Find something that works for you, it makes a huge difference
3) have you talked to your doctor? I'm not diabetic, but I would guess that s/he should be involved in advising your situation. Just to be safe, y'know
4) what time of day are you riding? Again, summertime in TX is a PITA. If your schedule permits getting out before 9am, it's worth it....
Keep it up!