Hello Swim Mom
If you'll humor me, I'll tell you my heat story . . .
It's November of 1987, and I am 31 and as fit as I have ever been in my life. We live in Austin, Texas, and it's the day before the Austin Half Marathon. Martha is going to pick me up at the finish, and I tell her, "I'll be in in 87 minutes, if I'm not at the finish in 95 I've collapsed somewhere on the course.
So I wake up on Sunday morning and turn on the weather channel at 6 am. It says, "good morning, the average high today is 70, current temperature is 70, humidity is 100%, and the dew point is 70. I grumble a lot, tell myself that I'm fit, go to the start, and take off anyway. I hit the 10 mile point in 66 minutes, and then completely implode. It takes me 31 minutes to run the last 3.1 miles. People are passing me like crazy in those last 3 miles, and when I see Martha waiting for me at the finish, the look on her face tells me I'm in danger. Probably the most discouraging result in a race in my life.
What you experienced is pretty typical for running in heat. The body does an excellent job heating itself in cold weather, but it does a very poor job cooling itself. The best weather for distance running is 40-45 degrees and dry. You are cold for a mile or so, but when your internal furnace kicks in you are comfortable during the race, and can really grind out a good result. You get really cold really fast after you finish, but you can push hard on the course.
When it's hot and humid your body simply cannot cool itself, as you are generating lots of internal heat, and the wicking of your perspiration cannot keep up. If it's humid as well, it's worse.
The solution is to slow down, take it easy, take fluids, don't take too much straight water, do take sports drinks with electrolytes. Too much water in hot and humid conditions can actually imbalance your electrolytes, and not to be scary, lead to severe medical conditions.
And yes, running early in the morning is better than 8 or 9 pm. It's usually humid as can be, but quite a bit cooler than in the evening. Of course, that means you have to be up really early. When I lived in Austin in the 80's I would do my long runs Sunday at 5:30 a.m., so I could be done by 7 or 7:30. I did my weekday runs at 9 p.m. but they never went past 7 miles.
I'm not sure where you live, but our Florida friends like Bill, Angie, Mel, Tracy, and Vick all have to fight this for over half the year. Honestly, I don't know how they do it. Hopefully they'll post their experience and advice. That's one reason why I always focus on the January WDW races, because the best time to train, by far, in Massachusetts in the autumn.
And running in the heat does allow you to learn what you can and cannot do. The 2007 and 2008 Disney Marathons were run in tough conditions, hot, humid, brutal sun, you name it. People with experience training in this stuff seemed to survive it better than us from cooler climates.
Good luck is all I can say. When it's hot and humid back off, take it easy, take fluids, especially sports drinks, and listen to your body. if you feel light headed get into the shade and recover.
Best
Craig