In reply to dj_brigham05: It depends a bit on where you're going to be running and how much kit you want to carry. If you're out for day runs, a 12 or 15L pack is probably quite a bit larger than you need most of the time. Something like a 8L pack might be more suited, or perhaps a bumbag if you get on with them?
The smaller OMM bumbag is quite good though not great if heavily loaded with food and water. I've got a North Face Enduro 13 sack (actually about 8L, I don't know why it's called a 13) which is very comfy and has a slightly longer back. Plenty of room for extra layer, jacket, waterproof trousers, camera, phone, hat, gloves, map and food etc. The Inov-8 Race Elite 10 is another very good if minimal sack - comes in about 100g! Neither of these have a dedicated hydration system which is good if you tend to just carry a bottle and fill up from streams, but not great if you prefer to carry all of your water with you.