• It can take a few weeks to acclimate to exercising in hot weather.
• You can be perfectly hydrated and still overheat.
• Fears of dehydration are vastly overstated. E.g. a study of elite Kenyan runners demonstrated that they drink very little water during training or events, end the day slightly dehydrated, and easily rehydrate in the evenings. Hence the "drink only when you're thirsty."
• Older people tend to lose some of their ability to regulate their body temperature. So you could be getting some heat exhaustion without realizing it, in situations that would not have caused a problem for you 5 years ago. (Note: I don't know at what age that typically starts to kick in.)
• Very few people need additional electrolytes, even during longer endurance exercise. Most people consume so much salt, that it's astounding that anyone ever gets hyponatremia. You also do not lose potassium in sweat. I.e. all that electrolyte stuff is essentially snake-oil.
On a side note, you didn't mention any solid food. Your body is still going to expect some sort of solid food after a few hours, so that could be adding to the mix.
I don't think there's a simple answer, it may take a bit of careful consideration to figure it out. I'd just commit the symptoms of heat exhaustion to memory, and be careful that you aren't overheating without realizing it.