First Half-Century - Followed by a Blackout!
I just turned 50, and have only been riding for about two months. But I've been riding 12-15 miles every day during the week, and 20-25 miles on Saturdays and Sundays, so I figured I'd try a half-century on flat terrain on Sunday.
I got up early enough to have a protein shake and 24 oz. of water an hour before starting the ride at 7am. Then for the 3.5-hour ride, I'd stop every 45 minutes for an energy gel and 24 oz. of electrolyte water. I also drank periodically from a water backpack over the course of the ride (about another 64 oz. of plain water).
I finished the ride and still felt pretty good - not gassed out or "bonked" at all. But then I bent over to give the cats some food, and when I stood back up, I felt lightheaded for a second and the next thing I knew I was waking up on the floor with a sore elbow from where I hit it on the floor. My wife heard the crash and said that I was already coming to when she made it into the kitchen, so I guess I was out for 5-10 seconds in all.
My blood pressure was 110/60 right afterward, and stayed there for the next few hours ('normal' for me is closer to 120/70).
I don't think the distance was excessive, considering I'd been riding 100+ miles per week for several weeks prior. But I guess the moral of the story is that I wasn't as ahead of the "hydration curve" as I thought, and that there's a huge difference in how much water I need for a 50-mile ride compared to even a 25-mile ride.