Getting older is a thing and probably you could have handled things better when you were younger. But 100+ F is dangerous for anyone exerting themselves. I don't mean to say no one should do it. Just that thermal management is much harder.

This sounds more like too many things stacked against you that day. The weather mostly, perhaps lack of adaptation to the heat and maybe some less than ideal preparation and route planning. And sure age isn't helping, but I'd bet that was not a primary issue.

I'm glad you listened to your body and took the ride offered. Else, you might have finished your ride in the back of an ambulance. As an endurance sport, we're all used to pushing though the discomfort, but there does come a point when that is a mistake.

I'd bet you can handle this same route another day. Better prepared, and maybe next fall when the weather has cooled.

Sometimes it's better to retreat and live to fight another day.