A backpack that fits me perfectly is uncomfortable and too heavy for my wife and "meh" for my son. You really need to go someplace you can try things on, see how it fits, how it wears. Plus, your idea of "hiking" and someone else's definition could be totally different in terms of capabilities.
My best advice--find a camera insert (usually about $30-$45) that fits the body and lens that you will likely be hiking with (most inserts can be adjusted but if you're bringing that 600mm lens for birding than it creates unique challenges). Once you've got an insert, go to REI or another good outdoor gear store and find a backpack that works for the kind of hikes you're talking about, will carry the gear you need and see if it will accommodate your camera insert. It will probably cost you less then buying a camera backpack of similar size and capability, will be more flexible, and won't have a "camera bag" brand on the back which invites thieves.
Additionally, most hiking backpacks will have straps on the top or bottom to accommodate a sleeping pad. The camelback bladder is the tricky part. I'm not aware of any camera backpacks that handle that (though I'm sure some do). Frankly, I'd rather have any water or bladder separate from my camera gear.