Originally Answered: Weight Management?
You haven't included your current height. I own a health and nutrition business, and have taught fitness, but cannot tell you much without knowing your current height as well as weight - in US pounds. Sorry, but I don't know what stone is! LOL
That said, the easiest way to work with some of your back pain is to strengthen your "core" which means that you need to strengthen your abdominals. Large framed people usually have too much stress on their backs due to weak abdominals, and this can result in back pain. You can remedy this with learning to do crunches for both your upper and lower abdominals. Don't do sit ups, as they simply are not as effective.
Crunches are achieved by laying on your back on the ground, feet flat on the floor, putting your hands behind your head and neck, preferably interlacing the fingers to support your head, and then, with eyes and chin facing the ceiling straight up, lifting your head and shoulders from the ground slowly - preferably on a "two count" (One one-thousand, two one-thousand up, one one-thousand, two one-thousand down). Start with 10 to 20 of these per day with the feet on the ground, and then cross one leg over the thigh of the other leg and do another 10, repeating with the other leg. Add a few each week until you are up to about 50. Make sure and stretch afterward by pulling your knees to your chest, arms around your thighs (or around your knees, if you can't get to the thighs) and rocking on your back gently a few times afterward, then pointing your toes out straight and reaching your arms above your head and reaching out as far as you can. You will feel the stretch in your abdomen, and may also feel your spine straighten out a bit.
It also will help you to look at what kind of shoes you are wearing for the walking. You want something on your feet that is made to give you maximum support for the alignment of your knees, ankles and hips, as well as good cushioning to absorb the stress on the joints. Hard bottomed shoes or sanndals will only aggrivate things.
I hope this helps and wish you well.