Biggest Loser Pound-For-Pound Challenge

I've been watching The Biggest Loser religiously this season. Right now they are doing this Pound-For-Pound Challenge, which people pledge to lose a certain amount of weight by the end of the event (May 5th--and you can't pledge more than what would be 2 pounds per week) and the sponsors will donate 1 pound of food for every pound lost by the people who enter. Since I didn't get around to entering until this week, my pledge is 20 pounds. I think that's a very realistic goal for 2 months.

BUT...

My weight loss has stopped again after my fill (and rebounded back up to about 413). I will be scheduling my next fill probably tomorrow, but also I REALLY need to start exercising. I haven't been keeping track of my calories, but I know that I eat less than 2,000 a day, and according to several websites that tell you how many calories you need to eat to lose such and such amount of weight, to MAINTAIN my current weight is like 4,000 calories a day! So how come I'm not losing anything when I'm eating half that at least? So I'm thinking exercise is going to have to start, which I was hoping to wait until I was back down to about 300, where the machines are a little more sturdy for my weight.