Today was the first really hot day here in New Jersey and I just did a brick workout in 94 degrees. 1 1/2 hour on the bike and 20 min run.
The bike was fine. I did 1 quart of fluids and 2 gels. When I came home and changed for the run I ate 1/2 a banana and drank a 1/3 quart of water.
However for the first time ever I did feel lightheaded after the 20 min run. I was supposed to do 30 min, but decided to stop after 20. My legs and lungs were fine but my heart rate was at 95% of max after 4.3k so I pulled the plug. Luckily I was only 1/2k from my house and a nice cool pool that really helped me feel better.
So remember: Go out there and do your workouts, but listen to your body!