Just too hot

I woke up nice and early and walked the 3 1/2 to 4 miles out to Flushing Meadow for the 50k race. I got there just in time to see the 100k race start. It was already warm and the humidity was high, but as we started the 50k race an hour later it didn't feel too bad. I was mostly running on my own, though there was almost always someone not too far ahead of me. The weather seemed to be getting quite hot and the Sun was feeling quite strong, but everything seemed to be going OK. At 21 miles I was on pace for a sub 6 hour time. Then I started to really feel the heat. I had been drinking a lot and making sure I was well hydrated, but then I started to feel a little dizzy, then nauseous and in the space of 3 miles I had lost it. As I completed 24 miles I knew it was a mistake to carry on. I discovered that 3 other runners had become sick and on medical advice, the 100k race had now been reduced to a 50 mile race. I spent a half an hour sitting in the shade and drinking lots of fluids and started to recover, but I had no feeling to go out and try to complete the 7 remaining miles. I had a problem with a heat related sickness about 20 years ago and have tried to be smart and avoid that problem since.
Once I felt well enough I packed my bag and walked the almost 4 miles back home very casually, getting more drinks on the way. When I got home 1 1/2 hours after I had stopped and after consuming about 50ozs of fluids, I finally went to the bathroom. When I weighed myself I was down 4 lbs even after re hydrating myself. So a lesson learned.