#1 - you did not fail. You did ride 56 of the 64 miles, so you succeeded above 85%. HONOR YOURSELF!

#2 - you can get good training tips from the abstracts of the medical symposium that is usually done before the Hotter 'n Hell 100. They discuss dehydration, overheating, heat stroke, etc - and proper training. It's a great conference - too bad they are skipping it this year. you can find info at www.hh100.org

A few helpful hints:
- first of all speak to your physician and get some baseline data. Temperature of course, and heart rate, but also some bloodwork would help. A baseline BMP and CBC, repeated post-long-ride, then with high heat/humidity, then post ride with heat/humidity) is a good idea, so that you know what is the range you play with.
- start training for elevated temperature gradually. Get out and ride during the hotter part of the day (remember a very good sunscreen and plenty of fluid), starting with short ride and possibly with some support available (husband, friend, etc). Increase the distance gradually, and learn what amount of fluid you need for different distances - and when you need to use electrolyte replacement or food. It is a very good idea to start with small goals and build up. I have seen experienced cyclists (racers) start heat training with a 10 mile ride, and increase the distance very slowly.
Ask around here and on the bike forums - a lot of people that ride the HH100 will reply with good advice.
Good luck!