I usually don't carry water when I run (I usually run under 8 miles at a time and make sure to almost over-hydrate before I head out and I'm sure to have plenty of water waiting when I'm done), but I have never tried just a hydration pack before. I have a Camelbak day pack (3 L) that I've used for hiking and long days out in the sun at horse shows, but it's far too big for running.
I might have to look into another, smaller pack for running. I can't carry a water bottle (or a shirt I remove, for that matter) while running without throwing myself way off.
“Hope is the thing with feathers, that perches in the soul, and sings the tune without words, and never stops at all.”
- Emily Dickinson