This is definitely a valid concern. However, I know I wasn't eating enough to do the ride on an other than fat-burning basis, which given what I have available to burn might work for a survival trek but not for the ready energy demands of cycling even at my slow pace. And the reason I wasn't eating was that under the conditions the food I'd packed made me feel sick.

In terms of heat exhaustion, for the troublesome hours I was mostly threading through tree-screened corridors within a hundred yards of quite built-up areas so wasn't really concerned; would have been a while before I could get collected by family, but plenty of opportunities to seek or get help. And at refills I probably dumped out almost as much water to replace it with fresh as I had consumed, so was never really low on that despite drinking about as much as I felt I could. Nor was there any shortage of other people riding, and it being a weekday many were a decade or two older than I, though in most cases not going as far. I think I might have at times been in danger of abandoning the ride, but never really in danger danger - avoiding the parts when I might have been getting close to that was why there were a few cases where I walked into the shade rather than riding there.