Does a hybrid pannier / backpack exist? From reading around on the internet I understand that pannier bags are comfortable to ride long distances with on a bicycle.

In my current rough travel plan I decided I also want to (hitch)hike a large part of the journey, so I would need a regular backpack (±70L).

To keep the costs down I wondered if there's some sort of hybrid pannier bag that not only can be hooked on to a bike, but also to a "body harness" in practice becoming a backpack.
I found some hybrids but these are small and have far less volume than a hiking backpack.

So does a system where I can carry multiple pannier bags on my back exist?
Yes. I own a north st bags convertible pannier and think it works great.  It hangs a bit low, so don't try and use it on low rider racks.

http://northstbags.com/products/woodward



Also see Richard Jones Convertible Backpack

http://www.convertiblebackpacks.us/



Also try WOHO bags "NINJA NINJA" convertible backpack:

http://www.wohobike.com/product.php?linkid=133