I know the feeling, 55 and 21 stone, my back goes into spasm as soon as I walk a short distance. Take it nice and steady and don't try to do too much at once, that way will see you in proper mischief pretty quickly. I have resorted to mounting my super lightweight bike on a turbo trainer, that way I can crank out a couple of miles no matter the weather.
You aren't allowed to use the kidnapped dog to pull the bike though.