You couldn't have been dehydrated if you were passing lots of clear urine. The main symptoms of dehydration are reduced urine output with darker colour. You were affected/ stressed by the conditions but not dehydrated.
Possibly not completely true, depends on state of your blood. Too much glucose in it will cause urination even if its hydration is less than it should be. The same applies to excessive salt and other chemicals, C2H5OH and any other diuretic. Doing hard cycling, i.e. with high heart rate as in near an individuals max, these are unlikely becasue carbo absorption through the digestive system is limited with most ofthe blood going to the muscles and consumption of glucose by the muscles is high. For lower efforts with innappropriate eating and lower muscle use, a higher glucose level is possible if ones insulin systems are struggling.
With correct training or ample caffeine, low effort cycling can be powered largly by fat. We don't need to consume any extra fat on a short event like an Audax
Eating fat also restricts digestive speed but not a problem if riding at lower effort where carbs are not required. I suspect using water or water with a small amount of salt if sweating hard would be best for hydration.
I find 80miles on just water(1L unless hot) is doable but am ready for the finish. I do this often as training. Sometimes the water has 30-50% fruit juice. The hunger bonk hits a few miles before the finish which is usefull for training for fat burning.
Mentally I require 1.5L with 30% fruit juice mix + ~5 muesli bars for a 200Km. If I eat too much I will piss more and need more fluid. If I eat too too much, normally a mental problem for me, I will start getting acid feeling in digestive system. When riding to a start I often end up going at max because I'm late, heart rate is 85% max for 1.5 to 3 hours(30-56 miles), I eat nothing during the ride out, drink only a few mouthfulls but arrive at the start in reasonably good condition. Here I believe I solve the mental "I want to eat syndrome" by worrying about the organiser may have dissappeared from the start by the time I arrive. I will have eaten some limited cereal or pasta and drunk coffee before departure.