Glad you've changed your mind.

I began a water fast a few years ago. Got as far as 14 or 15 days if I remember correctly. I have to say, unless I was an exception, you wouldn't be doing any gym work, because towards the end of the fast I was so physically tired all the time I could hardly stay out of bed. I think you get a certain mindset when fasting whereby losing weight is the most important thing. But fasting like that, and especially attempting exercise at the same time, is going to damage your organs, including your heart.