A serious but possibly obvious running question?

How much does running in heat and humidity affect running? I ran in 83 degree weather with 70 humidity and it was absolutely terrible! I usually can run 4 miles with little problem but today I could only run a mile or so and I couldn't push anymore. (I went on and off after the mile and a half, but for very short distances.) Does heat really affect your running performance that much? Or am I just a frickin wus? lol