Once it gets hot, I keep my Camelbak bladder about 2/3 full in the freezer all the time. Fill with water, blow air into it to keep the top of the bladder from sticking to the ice. When ready to ride, top it off water, throw it into the harness and off you go. Since you put water in you will not have to wait for it to thaw. Might be frozen in the tube when you first take it from the freezer but it will thaw in a couple minutes so that is not an issue.

When you take a drink from the tube, be sure to blow air back into the tube - which pushes the water in the tube back into the bladder. This keeps you from getting the dreaded mouthful of warm water when you go to take your next drink.

Since I started using this "system" I am still able to have COLD water 3 hours into a ride in the hot summer. About the only thing I really can't stand is drinking lukewarm water. Ever. Ugh!