One word: TRIATHLON - look into it, your steely resolve will be rewarded beyond imagination when the seemingly impossible becomes a reality (I'm talking baby steps - not Ironman next week).
I’m already a very good swimmer, even as portly as I am right now. Many years of swimming, in my youth, as a swim-team guy — endurance swimmer, not a sprinter.
So Triathlon might be neat to work on. Never been a runner though. Forced to, in the NY Guard, but I hated every sloggin stride.