What do you guys think of using backpacks for an overnight 35 to 40 mile tour? My girlfriend and I are going to bike the Leelanau trail and neither of use have panniers. We're wondering what sort of cons there are to using backpacks to hold a change of clothes, camera, some personal items, snacks and a bit of extra water? The backpacks we have are technical hiking packs from REI (traverse 30 or something) and have straps across the chest and waist for stability as well as some textured material so that it doesn't press flat against your back.

Thanks for your help