1mor,
heat and humidity can really take a toll. I'm 56 and I rode the other day only 20 miles but 85F and sunny, no breeze, high humidity, and some steep hills. I really got wiped out. In fact, I had to rest the next day- i think i got some mild heat stroke.
Usually I can do 30 or 50 miles and feel ok. I don't think it would have bothered me quite so much a couple years ago, but i do notice the heat effects me a little more now than it used to even though I'm in pretty good shape. I'm trying to listen to my body more these days and not push beyond where I would start feeling ill. But then, you are younger than I!

50 miles at 85-90F?.... I would have been ready to throw in the towel at that point too! I think you did great!