I carry a 40oz HydroFlask in an Arundel "Looney Bin" adjustable bottle cage. I call it my drop tank. It's not so much for the extra capacity, though that is nice, but for the "keeping it cold" factor. Like you, I drink a lot of water during the day; usually an ounce per pound of body weight, which for us works out to quite a bit. With the drop tank full and a 21-24oz bottle in front, I have enough fluid for 3 hours, or about 45 miles, at 60-85F temperatures.
Did 84 miles on July 4th, though, and went through over three gallons in those six+ hours. 60oz in the first three hours (below 70F), and the rest in the following 90-105F. But urban riding makes for easy refills. $1.50 for a gallon of ice-cold water at AM/PM or 7-11? Yes please. Stop, refill, and drink as much as possible before handing off the rest.
Camelbaks and their ilk are appealing, but I dislike covering my back. I prefer to expel water than to let it ride piggyback, so I'm sticking with my bottles for now.
I've heard of people in the country driving out along their route the prior day and stashing 2-liter or gallon jugs along the route. I like this approach as well.
