I have run a 10k on July 4 in the town I live in 6 times in the last 12 years. The race always starts at 8:30 am, and the weather is usually warm. 12 years ago with temps in the mid 80s I ran the 1st 5k in about 19 min and the 2nd 5k 6-7 minutes slower. A couple hours after the race I had a mild bout of heat stress, after drinking 2 qts of liquids. Moral of the story is exactly what Ryan states: Listen to your body and do not overdo it. The heat can easily stress you if you push too hard. But slow workouts in July can still pay dividends in the fall.
On a related note, even though I'm about 4 weeks away from being able to start running again and chomping at the bit, I did not feel bad at all missing the 10k yesterday. My 40 minute bike ride was bad enough, and I was home before 9. Cannot imagine those who raced when the heat index was 95 @ 9:00 am.
Be careful out there…