When it's hot like this, my rule is, if I'm going more than 8 mph I don't pedal. It's slow, but it keeps me from overheating.
I read an interesting article about the physiology of heat acclimation (sorry I long since lost the link). It involves increasing red blood cells because of the extra blood needed as it's all shunted to the periphery for cooling. According to the article, athletes acclimate within days, the rest of us take a couple weeks or more.
2009 Trek 7.2FX WSD, brooks Champion Flyer S, commuter bike