Heat is horrible. It stinks if one's not acclimated (which has been rough on those of us up North, since it's only just recently that we're getting days in the 80s), it sucks if one is living in an endless string of hot days. Maybe 6 weeks ago DH and I had planned on doing a ride of ~70 miles. Wouldn't have been a big deal, except that day ended up being suddenly in the low 90s with high humidity and wind. Felt like a convection oven. I started to get those chilly tingles by about 12 miles in, even though I had already guzzled a 24oz bottle of sport drink and taken some Endurolytes. My body totally freaked-out. So out ride ended up being half of the planned distance. So frustrating.

I suspect your fitness is better than you think. And you were wise to not push yourself into the hospital. Heat is nothing to mess with.