UniChris

I've personally ridden centuries (and ridden them a slot slower than you would, right through the hottest part of the afternoon) wearing a backpack, but I'd still argue for getting as much on the frame as you can.

Two bottles on the frame, two bottles behind the seat post should get you pretty far. Could you just put two 20-24 oz spring water bottles in your jersey pockets for the first 20 miles of the 68, to augment 2 on the frame and 2 behind the seat or even on the handlebars? You'd actually be hauling more than a gallon at the start of that segment. (Or put bottles on the forks touring style, if the dirt means you'll not be going at speeds where aerodynamic matter as much)

For many of my rides I end up stopping at a gas station or convenience store and I just buy a gallon jug of water to refill my bottles. The more people you have, the more sense that would make, ie you might end up with 2 gallons spread among three people or whatever.

I have thought about the stash thing, but never done it. If I was going to do it I'd use factory sealed bottles or jugs (to know if some joker messed with them), label them, and hide them in the shade. And I'd only do it for a case where I had a backup plan I could use, but don't want to. For example, both the centuries I've ridden and the one I'm planning required repeating the initial segment to bump up the distance. It's occurred to me that I could hide a gatorade bottle or two on the initial part, and pick it up on the repeat - should it be gone there is a gas station that I've been to once that is way down a hill I don't want to descend only to have to climb back up again. Next one planned has a grocery store at the start so I'll just duck in there before setting out for the second time - actually kind of tempted to set out with only what I need to do the initial segment in early morning conditions instead of a full load, but it's flat and I'm used to hauling more and really dislike running out of water with any distance remaining. Real question might be if I take my usual frozen sandwiches or try to buy food then, will probably stick with my usual.

Or this one could be fun... Get yourself some box wine and enjoy it. Then thoroughly wash out the bag and put it in a cheap string backpack. Disposable hydration pack, or perhaps compact enough you could roll it all up and stuff it somewhere when empty.