So, me being me, I went online when I got home and checked the stats. While it was only 75 degrees out, the humidity was 86% and the dew point was an uncomfortable 70 degrees. To reiterate, a dew point 60-69 is perceived by most as somewhat uncomfortable, 70-74 as very uncomfortable, and above 75 as being extremely uncomfortable.
Today the New York Times has a good article about how to avoid heatstroke. Definitely worth the short read. Some key takeaways include slowly acclimatizing to the heat over the course of days or even months and stopping if you truly feel bad.
Keep cool and keep running!