Hey there Sookie. By chance are you doing water enemas during your fast (2 liter bag)? In my experience they make a day and night difference in how energetic i feel. I make a high priority of doing one in the am and one in the pm. The tremendous amount of toxins it eliminates each time keeps your body from feeling so weak.
I must say as well that in my experience "2 a day" brisk walks of 30-60 minutes each make a nice difference for me.