You can improve your ability to cope with heat in only one way I'm aware of, and that is to train in the heat. Here in central Texas, you simply can't avoid heat in the summer so you continue to train. It really works. If you only have occasional high temperatures, it's not realistic to expect that you will be able to perform as well. Do your best to survive (the things you already did) and live to ride a cooler day. I'd say you did pretty well.
-soma5