I live in GA and it's hot a lot here. I find that on a long day, 100 degree air moving through my mesh jacket, mesh pants, and mesh gloves is nice but still 100 degree air. This is very little cooling effect. It's hot and I dehydrate constantly no matter how much I drink. If you do a five hour ride in that heat, plan to drink at least a quart an hour. That's what we always did when backpacking in 90-100 degree heat. If I had all day and could stop, nap, rest, etc. maybe I'd ride. If you need to be presentable and ready to work when you get there, take the car.
Ron