Hi Rosie. Don't think there's that much of a vacuum at the moment - I have a few bags that I use for work and back depending on how much rubbish I need to carry from a bum bag if everything's at work to a 25 litre pack if I need to take a pile of papers and my shoes. I don't mean to discourage in any sense, just not sure your initial assumption is correct.
Now if you could design a bag to carry a DSLR and lens safely and comfortably on my back while running, then I'd buy one in a heartbeat. I haven't been able to find anything yet that fits the bill, they either carry the camera safely (regular camera bags) or comfortably (running bags), but not both.