This explains a lot

The heat index is calculated with the assumption that the person is standing in the shade.  No wonder being in the sun makes it so much tougher!

What difference does being in the shade make?

Quite a big one. All of the National Weather Service’s charts for calculating the heat index make the reasonable assumption that folks will look for shade when it’s oppressively hot and muggy out. Direct sunlight can add up to 15 degrees to the calculated heat index.

I'm not crazy!  Thank you, Mental Floss.

Incidentally, yesterday I ran 4 miles without stopping.  Yay!  The weather was still a bit nicer than usual.  Went around 9am.  I had planned to go this morning, but was feeling worn out from a yoga class last night.  Instead, I'll go either tonight or tomorrow morning.