I am Sam wrote:
Put it another way, if a really fat bloke, that was once skinny most likely, came to me and said "I will never ever be able to run sub 20 5km, because my PB is 40min", what should my answer be?
I don't know what your answer should be, but here is how I would answer.
"You may or may not have the potential to run sub 20 5k. But you will never know unless you try. And trying includes losing significant weight in your case.
You may never run under 20min no matter how hard you try, but losing weight is good for your health. So is engaging in regular cardio exercise. So if you can embrace the process, it will be rewarding for you regardless of the outcome."