This is why people train at high elevation, they're essentially running in the heat of the day all the time compared to someone at sea level when you look at the different density altitudes. The mid-day run isn't hard because it's hot, it's because the heat means more moisture and more moisture means less oxygen available.
I.E. The density altitude here today is 2,700 but I'm at 5,000ft altitude so I'd rather run in the afternoon to get closer to the actual altitude. Running in the morning or evening wouldn't benefit me as much for training being at higher altitude. I've had some runs in the end of summer when the density altitude was around 9,500 at 1PM. Those were tough runs.