We really hate summer riding in Texas (hot & humid) because we can no longer afford not to start at sunrise. Last summer we hit the road at 6:45-7:00 if we were doing a long ride. I take a lot of naps after rides! 
You will acclimate but I find it easier to adjust through the course of the ride. I cannot start out at 10 am since the sun gets really intense by about 10:30.
Amanda
2011 Specialized Epic Comp 29er | Specialized Phenom | "Marie Laveau"
2007 Cannondale Synapse Carbon Road | Selle Italia Lady Gel Flow | "Miranda"
You don't have to be great to get started, but you do have to get started to be great. -Lee J. Colan