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.