There are no aid stations on training days-and who wants to plot their runs around water fountains? This best-selling vest carries 2 liters of water for runs of 2 hours or more-and its air mesh panels keep things light and breathable (you’ll be sweaty enough already). CamelBak has also included compartments on the shoulder harnesses so you can access your essentials without slowing down: a sweat-proof compartment for your phone, a stretch-mesh envelope pocket for gels, and storage for two extra water bottles. Above all, this pack is designed to fit comfortably without bumping around. The dual sternum straps offer more stability, and the top strap is made of elastic for a snug fit that won’t restrict your breathing. And thanks to the adjustable harness and drawstring compartments, you can cinch both the pack and reservoir down as your load gets lighter, so you won’t have to put up with a bouncing, sloshing pack at the end of your run.