While this is advice is for all marathon runners, it is particularly geared towards the obsessive ones that feel guilty when taking a day off. I place myself into this category.
Thursday night I was really tired and could barely drag myself out of bed on Friday morning. Not to mention, I had run for an hour on Thursday morning and had a bit of a sore knee. So I took Friday off.
All better today! My knee problem is gone. My energy is back. I ran this morning with a spring in my step.
Take a day off when you need it, even if you had planned to run that day. Your body is trying to tell you something. Don't feel guilty about it.