For long runs, it is extremally important to maintain a healthy hydration level in your body.
The consequences for not doing so range from cramps, mild to severe headaches resulting from dehydration right up to the potential body collapse.
If you are not able to carry water during a long run, at least be able to access water from potential sources or refreshment points during the race/run.
In answer to your questions how to carry enough water for a long run.
Here are three sound suggestions:
This will be your first source of water, a standard handheld bottle will hold at top end around 20 ounces, these bottles are designed to be comfortably carried, the bottle case will/should have a pocket so you can carry additional nutrition such as an energy gel or electrolyte tabs. If there are water stations, in the long run, you can refill the bottle quickly.
Multiple carry-Bottle Belt
These belts give you more carry capability, you can even vary what you carry, energy fluids and water, your carrying capacity will range from and depending on the belt 2 to 4 bottles with 8 or 10-ounce carrying capacity. These belts distribute weight around the hips so they are comfortable to wear.
Hydration Vest or Pack
This option opens up hours of ongoing running without the need to stop and fill a bottle. Generally, you get a water carrying capacity from 1.5 liters up to 2 liters-50-70 ounces. You will be hands-free and this is a sound way to carry a lot of fluid. If you're on a long run and don't want to carry a bottle which will need refilling or wear a belt, this is a good option.
Enjoy your run and remember, consider your options and be sure to stay fully hydrated during your run.