Everyone is missing the real problem, it's not the heat or humidity.
They are running back and forth for two hours next to a busy highway in heat and humidity.
Florida does not have car inspections or vehicle enforcement of any kind.
There are hundreds upon hundreds of pickups using electronic (or physical) exhaust bypass because they are never checked, never caught, they think it's fine to save a few dollars per tank while spraying their poison everywhere.
Even without bypass, at highway speeds the diesel particulate in exhaust are toxic as hell.
Florida cities have some of the worst air quality around, no sane person would run next to a Florida highway, certainly not at a high level effort.
But hey don't take my word for it, it's been studied by so many people they made a word for it "TRAP" (Traffic-Related Air Pollution) google it yourself
https://google.com/search?q=runners+TRAP+pollution
https://scholar.google.com/scholar?q=runners+TRAP+pollution