I ran my five-mile rest run today and enjoyed being out. My wakeup heart rate was 55, a bit higher than normal, but I felt fine. I was out earlier, and the temperature was 75 (F) when I left home. However, I spent half an hour watching two guys fly a RC dirigible, so the temperature was probably about 80 when I started running. It was about 90 when I returned home.
I think I'm getting used to the heat. During my run I was hot and sweat a lot, but I didn't seem to get tired due to the heat. I measured my pace during the last half-mile at 13 minutes. A slow pace, but fine for a rest run in the heat. I drank about 24 ounces of water during the run.