I felt like I was going to faint Monday on my 3.5 mile run. I was sweating like crazy, breathing hard, and a little light headed. Sure I gave blood last week and haven't had much iron to eat since then, but wow that was still harder than I expected! It really isn't a surprise when I think about it, increasing the volume of training, of running, that I do during the summer is very difficult. Because of the increasing heat and humidity a four mile run in 80°F and 80% humidity will feel as hard as a four mile run in 75°F and 70% humidity did last week when I was in worse shape.
Moral of the story, don't get down on yourself when you are out there dying while exercising in the summer, take it easy, stay hydrated and get your electrolytes. When the air cools down in September and October, you may be in the best shape of your life.