I've been reading a book this weekend - "The Complete Book of Road Cycling Skills" written by Ed Pavelka. It was published in 1998, so the advice may be dated, but, how much could wisdom concerning hydration change in that time?
The recommendation in the book is to make certain you prepare ahead of time by drinking plenty of fluids before you go out - then consume 4 - 8 ounces for every 15 minutes you ride strenuously. I'm guessing this would apply more to riding in hot weather than in cold, but, that's just my guess.
It sounds like a lot to me, but, then, thinking through a ride I took yesterday, I rode 60 miles during five hours. I had purposely added two tablespoons of salt to two 8-ounce glasses of orange juice I drank before starting my ride. At 30 miles, I drank two 32 oz bottles of Gatorade (not because of any conscious effort to conform to this book - just because I was thirsty, had stopped at a convenience store to take a break, was busy watching some young families fishing in a creek, and, during the course of that break, downed one bottle, then, still feeling thirsty, decided to have another).
I felt a bit slushy as I started back, but, within a mile, felt warmed up again and finished the ride without feeling uncomfortable or thirsty.
I don't relate this experience as an example of what you should do, just thinking through my fingers that, although 4 - 8 ounces as recommended in the book sounds like a lot of fluid to me, when I do the math, it is clear that I, indeed, consumed 4 ounces for every 15 minutes, even though I didn't space it evenly. What I did is probably poor management - but I did feel very comfortable throughout the ride.
I'd be curious to know what folks have to say (especially someone with some medical training) about my consumption of salt before the ride.
When I was in school, I took a job for two summers that involved working inside a very, very hot building working with very hot materials. Until some of the older hands wised me up to salt tablets, I would feel sick and ready to faint after the first two hours, a condition from which I would not recover for the rest of the shift, no matter how much water I drank. One day, an old guy finally took pity on me and showed me where they kept the salt tablets and how many I should take. After that, I could perspire to the point where I had to dump my perspiration out of the boots we wore three times a night - but I never felt the least bit uncomfortable physically.
I've been practicing that salt routine whenever exercising strenuously ever since.
Good luck on your event. Sounds like great fun.