My energy is fickle, it comes as goes, as Im sure it does for everyone. And really, I don't know how sometimes I have the energy to do such things. Last time I did a long fast I was so weak I could barely move. Every fast is different. Sometimes I don't have the energy, though, but I've been lucky, most days I am able to. And sometimes I just force myself to....But I need to do some sort of exercise during the day or else I can't sleep at night due to excess energy, so I find that's it worth it to push myself a little.
The things I have the most trouble with actually is sitting in class, or sitting at the computer typing an essay, because my muscles ache. That's my biggest problem.