Don’t worry, it’s only awful when you’re outside. I’m quite a bit lower in latitude, but I remember temperatures reaching 90 in December a little over a decade ago. Most of the summertime is spent indoors unless you’re running (early morning, late night) or at the beach. Spring/Fall are prime times to do outdoor activities. For better or worse, northerners generally acclimate by the next year (sometimes two, depends how cognizant ((shocked)) they are).
The native Americans thought the southern lands were well and good far before the Spanish got here in the 1500s. Most everyone else since has had the same thought as you if they didn’t grow up here. I will say though… taking a flight from dry states (Texas) back to Florida is a shock getting off the plane. Easy to forget how much water is in the air. Basically mer-people around these parts.