Being from SE Florida, hydration is always on my mind, as well as my local cycling friends. For long- endurance events, such as a century ride, I need to hydrate DAYS before the ride-- usually beginning Monday or Tuesday before a Saturday event. I personally have very little thirst during the week, so I need to force myself to drink even a liter of water per day. If I don't do this, my performance is decreased during the weekend ride. Hydrating the day before is not enough. I've forgotten to hydrate a number of times and try to do this the day before... it never really works well. When I have appropriately hydrated 1-1.5 liter/day a few days before the event, I ride a bit faster and with less fatigue. Yes, this is anecdotal, but I've repeated these results, good and bad, repeatedly.
Before my ride I drink a mug of coffee about 90 minutes before the ride and 8 oz of water about 60 minutes before the ride. During the ride, I drink Infinit Go Far in a 24 oz bottle every hour during the summer extreme heat and every 90 minutes during the cooler months here in Florida. It has all the electrolytes and carbs and amino acids used during a ride. I use a custom Go Far blend and cut back a bit on the carbs. The original formula gives me a bit of lead belly after my fourth bottle. I do this by drinking ever 10 minutes while rolling on the bike. If I get thirsty, I drink a few gulps of plane water that I keep in my back jersey pocket.
I'm a 57 year-old man with BPH (benign prostatic hyperplasia)-- an enlarged, non- cancerous prostate. Yes, I pee more often than I used to, but during a ride like you've mentioned 45 miles/ three hours, I'll stop once, maybe twice to pee. Perhaps the Florida heat causes me to sweat more than most, I'm assuming reducing my need to pee.
After an endurance ride, I'll drink 8oz of almond milk with a scoop of protein powder, and then a glass of water. Later in the day, I'm more aware of my thirst and top it off as needed.
Before I started working with a coach, I did experience hyponatremia during a century ride on a hot day. I drank plenty of water but was not using any electrolyte powders. I started going into heat exhaustion, cooled off at a rest stop, indoors for 20 minutes, then got back on the bike for another 16 miles when I reached the last SAG stop. I began to go back into heat exhaustion again and really feared I'd go into heat stroke. I was also having difficulty concentrating during a conversation at the rest stop. They ran out of ice, so I requested to be driven back to the finish line. It was the smartest thing I did all day. I later realized I was hyponatremic when I felt better after some salty food, and I had peed large amounts of clear urine 5 times in two hours after the ride.
I hope this wasn't too long.