I hit over 120 F a few times riding between southern California and southern Arizona in August. What I can say is that as long as you have water it isn't really too bad in the dry heat. I would prefer 120 in the desert over 95 in Alabama. Water and sweat are the limiting factors. Ride too hard and you will sweat out faster than you can drink water. What I would do is at every mile marker, take a swig of water and spray down the back of my jersey and tops of my thighs. Spraying yourself with water really cools you down a lot and it saves some of your sweat which is harder to replace (because of salt and the rate your body can drink water). If you run out of water, take it easy. If you exert yourself, you may seize up your legs with muscle cramps and then you are stuck with no engine.

Self supported, I carried about 3.5 L for stretches of up to 50 miles or so. and only ran out once because a municipal water treatment station out in the desert I was planning to refill at was fenced off and refused me entry or water. Jerks.