I want to warn you that I don't recommend a 20 day fast for anyone unsupervised. Secondly, you don't have sufficient experience or preparation to do a 3 week fast.
I was hoping the bleek picture I painted for you would get you looking for other ways to do this.
Look at the posts below of the people who have lost and regained. This is called yo-yo dieting and Water Fast yo-yoing is the hardest on your body of all the yo-yo diets around.
You could run into serious health problems.
With that said, i'd be interested to see your posts as you go along. Hardly anyone can make a 20 day fast. Fortunately the body rebels and they get so sick and scared they have to break the fast. You are undoubtedly toxic, that's what overweight is. It's not a good idea, but your on your own.