Crunchie, just to echo what Mal said I'm eating 1500-1800 calories a day and exercising 2-3 times a week by walking 7 blocks--this is a very low danger/low impact diet. I've been taking off 6 pounds a week doing this (outside of the two trips). I have no idea what to expect, but I have been through some rather long and time consuming weight loss before. Eventually I will slow down and hit some plateaus, but at my size it is going to come off fast. That's why I set the ridiculous goal of 66 pounds for summer. When I was on the other side of 25 I took off 18 pounds in one week by doing Jenny Craig at 2,000 calories per day.
I am against weight loss as a competition because it is unhealthy, but this kind of challenge gave me a little extra motivation that first week not to blow it because I was part of a team. The people on here do not give you a free pass on too few calories--its one thing I like about them.