I agree that individuals adapt to the heat. I've experienced it personally and repeatedly. But you drastically over estimate the rate at which human evolution would pass on that trait. In order for natural selection to occur you first need for a random genetic change to make people function better when they're hot. Then you need for that specific thing to result in a survival advantage that influences one's opportunity to reproduce. Then that change needs to take hold and spread throughout the population.

You're already on shaky ground but now combine that with the fact that your hundreds of generations involve human beings that more and more are not genetically evolving anyway anymore. Our ingenuity has increasingly made us capable of surviving regardless of our physical condition and other traditional drivers of your likelihood to survive long enough to reproduce. For example being able to build fires means it doesn't matter as much that you might freeze to death without it. But now we have houses and beds and hvac and cars and handicap access and it matters less and less what your physical abilities are or what kinds of genetic defects you suffer from that might make you less resilient in the wild, where you and nobody else goes except to visit briefly.

Thankfully genetic engineering will step in and give us a way to continue evolving, this time in a more self-driven way.