There's no time I like more than when I can finally exchange my YakTrax, long-sleeved shirts and pants for a t-shirt and shorts to hit the roads. After a long and very cold winter in New Hampshire, it's finally time for spring and summer.
But what that means for me is that I need to be careful about hydration. During the winter, I can do 6-8 mile runs without taking water with me. In the summer, I at least need to use my fuel belt. If I run over 10, that means I take my Camelbak. The rule that I stick by is that if I get thirsty, it's already too late, so I try to hydrate early and often.
The same 20 degree rule that you used in the winter also works in the summer. Your body thinks that it's 20 degrees warmer than the air temperature, so address your clothing appropriately. You don't want to over dress and get overheated.
The other thing seems like common sense: listen to your body. If you need to stop to walk, do it. If you need to slow your pace because your heart is beating out of your chest, do it. I've stopped a number of times and I'm always glad I did it. Your body is not just fighting the distance and the speed, but it's also fighting the heat and humidity. No reason to be a hero.