For day hikes with camera gear, food, water and rain gear I'll use a Lowepro Flipside 350 which easily carries a 150-600, 200-500 or 500mm PF attached to a camera body plus extras like another lens or extra day gear. I'll usually also have a Black Rapid strap that I can attach to the lens foot to carry the camera at the ready but don't like that when I'm hiking long distances as I still have to keep a hand on the camera and lens to keep it from swinging as I walk.
For actual backpacking I don't try to keep the camera and long lens at the ready but have it packed away inside. I might carry a smaller DSLR or MILC with a shorter lens mounted on a Peak Designs quick release attached to the waist belt or a shoulder strap but I find a big lens gets in the way and those rapid fire wildlife shooting opportunities aren't as common as I often think they'll be. I'm more likely to pull out the big lens when we stop for an extended lunch or at the next camp and prefer to shoot in the mornings and evenings when light is best anyway.