This isn't the exact belt I have, but similar.
Each bottle holds 10 ounces of water which can get me through about a half marathon. When I'm out training I will stop at my parents' house or the library and refill my bottles when I need more water during a long run. In my last marathon, because the water stops were every other mile, I wore the belt and had Dave meet me at the half way point and hand off full bottles to me.
The belt also has a pouch that can hold quite a bit. I have fit 7 gels in there, which is plenty for a marathon. Mine also has a small compartment on the front flap that I will put my house key or some chapstick.
One thing I love about the Nathan pack as opposed to other packs I've tried is that it fits and does not bounce around. I love this! Despite the weight that is on my hips, I don't notice it, it doesn't get in the way while I'm running, and it doesn't seem to affect my pace or gait either while I'm running.
Do you have a favorite piece of running "equipment"? What is your go-to hydration system?