I love my Osprey pack http://www.ospreypacks.com/en/group/...ydration_packs . The chest and waist belts will keep it solidly affixed to your back. Storage is secure and, if you want to carry more than its capacity, it's time to start thinking about a rack. I don't think you'll be comfortable riding 5 miles each way with too much more weight on your back.

Good luck.