I have never seen a dewpoint much above 80. I can't imagine where you would see that.
This morning it was 77. I did a 7 mi tempo run that ended up being about 20-30 sec per mi slower than the same effort level in good weather. You can train and race in high humidity/dew point, you just have to adjust your times. 79 is the worst I have ever had.
For me, anything below 65 is fine. Certainly, under 50 feels much better than over.