I too started out 2 stone heavier so had the same problem.
I'm no expert, can only report on what I did but think you're on the right track.
Start with the distance you know is sustainable for you, then build up in manageable steps, e.g. 10 mins or 1 mile, whatever your preferred medium. You could still complete a slightly longer weekend run, rather than your usual long one. As you know yourself it's a mental rather than physical block, once I could manage 4 miles without a break I never walked again and the distance came easily after that.