I just finished my forty day Water Fast last Saturday and I must tell you that the first week was probably the hardest for me. After that my energy level just kept getting better and better. During the last twenty or so days I was walking from one to three miles, three times a week, and I went skiing twice during the last week. My energy level seemed to be through the roof. My stamina, while skiing was a little suspect, and I had to stop sooner in the day then I normally would have.
It got better for me the longer the fast went, maybe it will for you too.
Best Wishes to you.