My first bout with hypothermia was in 80F weather, doing a very active swimming test. And I don't think I'm the warmest weather hypothermia case on record, by a long shot.
Hardening up doesn't prevent hypothermia. Or heat stroke. It is pretty easy to end up in conditions where your body's thermostat responses can't maintain your core temperature. If you can't maintain core temperature and don't get treatment fast... you die. Heat stroke, you die because your nervous system got cooked. Hypothermia you die because you run out of fuel reserves (possibly with the fun additional benefits of freezing solid or massive shock).
Not something I'd wish on anyone else. I got through it because I knew the first aid, and got rewarmed quickly. Even the early stages of hypothermia really mess with your head. It's not a fun time, and I do not recommend it in the slightest.
And joy of joys, once you've had hypothermia or hyperthermia once... you're much more likely to develop it again. Be kind to the EMTs, don't court it.