That would certainly help explain why I end up in the med tent after so many of my summer races.

There's no mystery about what happens.

 

Your body cools itself by convection.  ... 

 

As you get older, your heart won't beat as fast and your blood vessels become stiffer and won't dilate as much so you can't regulate your temperature as well.  And there isn't a damn thing you can do about it.

Actually there IS one thing you can do about it - lose some body fat. My body fat is in the single digits, and a fair number of the blood veins in my arms, legs and a few on my gut are at the surface, bulging and visible through the skin, and quite effective at cooling me off when needed. Most people  who complain about the heat have no visible veins in their arms or legs because they're all hidden under a blanket of insulating fat. Fat is great for cold weather, but bad for warm weather. I actually ENJOY the feeling of running in the heat - my muscles work so much better over 60 F than they do at under 50 F. And I usually pass a lot of people in warmer weather runs.

And one more thing, whenever I run races in warm or hot weather, a high percentage of the other runners will be wearing stupidly hot clothing choices that help hold in or even create more heat. Wearing cooler clothing and in lighter colors should just be common sense and not even have to be talked about.

I am way happier running a race of any distance at 70 F  than 40 F.  Your mileage may vary . . .

I do better (especially mentally) in warmer conditions (not hellish, though), than in colder too. But for a half and a full, 35-50F is best in the end. Although for a full, I have done not bad in up to 72-74F. Less than 30F and more than 75F is difficult for me.