I've been using a camelbak. I could carry two 32-oz bottles in my trunk bag, plus my two bottles in cages, and that would last me 30 miles in the heat. But, I'd get dropped off my group when I stopped to swap bottles around. The camelback eliminates that, and is also much better at keep stuff cold than the insulated bottles (fill it with ice, then water). And it's pretty common with the rando group here.