Keep in mind that the larger you are, the more calories you burn. If you are in the habit of eating X calories per day, your body naturally reaches whatever mass is necessary to burn X calories per day on average (assuming constant activity level).
Let's say that today, I'm at an equilibrium where I am eating 2800 calories per day and burning 2800 calories per day and my bodyweight is constant. If I put on 100 lbs overnight, just because I have more mass, I will now be burning something more like 3,500 calories per day, all else equal.
So the day I wake up 100 lbs heavier, I also wake up with a calorie deficit of 700 per day. I'm already primed for rapid weight loss before any diet and exercise changes. It would only take very modest lifestyle changes to push the calorie deficit to 1000+ per day (though I might need to switch to a lower impact form of exercise than running for a while). If I cut the calorie intake to 2000 per day and start working out like I'm in college again, and I'm looking at a calorie deficit of 2000+ per day (not sure how sustainable that would be, as I may be too damn hungry to keep my intake that low).
Not really sure what my specific plan would be. Just making the point that reasonably rapid weight loss is going to be pretty much automatic.