Hi Andi
I thought the exact same and started by walking then running for a minute at a time... You will be surprised how quickly it builds up! Also bought a NIKE + Sports Kit for my Nano and have downloaded a couple of personal trainer training sessions... Must admit i don't run the whole way but run more than walk as i am building gradually as i have put in for the Marathon!
I'm also 17.5 stone with a E cup size chest, but a good sports bra has made a difference!