Congratulations on the weight loss mate, keepi it going. As RDS says below, maybe look to do some exercise that's less hard on the joints than running, until you get the final 3 stone off. As you get closer to the target, you can start upping your running. 3 stone is still quite a bit to be carrying if you're hitting the roads, imagine putting 21 bags of sugar in a ruck sack and jogging around with that on, it's all going to impact on your joints, ie back, hips, knees and ankles. Once you get down to your target weight, as you rightly say, start at a relatively easy distance like 3K, then increase that gradually up by about 10% a week. I properly knackered my ankles and back a few years ago by trying to run too much before I'd lost weight, but now with the wegith gone it's much, much easier. Try to mix in some strength and conditioning and mobility work, it all helps massively.