I take a lot of long runs that are single loops or 'out and back' runs, depending on the route. I have to carry whatever hydration and nutrition I need on me and tote it through the course of the run. There's a fine line between packing adequately and weighing yourself down with needless crap. I used to pack a lot more than I do now. If I'm doing anything under a half-marathon distance, I will take one bottle of Gatorade. 20 ounces or so. I might take 1 GU, but other than that, it will just be the sports drink. For longer runs, I'll pack more.
There are a couple of options for carrying your hydration and nutrition. I've got a Nathan waist pack bottle holder that I've finally gotten adjusted correctly over the course of about 3 or 4 runs. The pack holds a standard size bike bottle (it came with a bottle as well) and a gel flask. It has a zippered pocket for more fuel, car keys or a cellphone. I've got a Fuelbelt with 4 x 8 ounce bottles and a pouch. I've also got a Camelbak-style pack that holds 2 liters of fluid and has all kinds of pockets. I'm going to give it a try on the next run and see how much it moves around. It might be the pack I use in Vermont this year.
The other thing I've done is just carry a bottle in my hand. I'll swap it back and forth over the course of the run. Summary - there are all kinds of options out there for carrying crap while you're running. The trick is to be comfortable with whatever you're using because if you're not, it will be a slice of hell over the course of a run because your pack will start bothering you and there's not much you can do about it! If you're trying something new, start with a short run and see how it goes. Move up to a longer run once you get things tweaked. A final word of advice - NEVER make alterations to the equipment you are using right before a big race. Stick with what you're used to training with.