Acute heat exposure, like 45 minutes where sweat is starting to accumulate on your skin, causes capillary growth and other adaptations some weeks later. If you don't have those adaptations already when exposed to heat, and you're weak, you're going to be in trouble.

Are we distinguishing eustress and distress?

I guess this explains how I could play tennis for two hours in the sun in 35 degree heat (35 C in shade). I guess I'd better not try it again now that I haven't done it for a while.