The Illness excuse

I'm training and I'm sick--this is the most recent of my predicaments.  More likely, I'm sick because I'm training (and because the weather in Boston is dreadfully unpredictable year-round!) But let's not blame the weather. What's a girl to do?

This week, though it's supposed to be an easier one in my training cycle, I am packing in the miles. ( I missed an 8-mile run last weekend and feel subsumed with guilt. Coach said: Run it Wednesday.) Anyway, yesterday, I snuck in four miles before track--probably a bad idea--and today, on the advice of a marathoner friend ( and also because I felt "okay") I put in another 5. Five laboriously slow miles that gave me enough energy for the rest of the day.

So, now I wonder: How does illness alter training? How do I know when to "take it easy" or "take a day" without the obvious critical symptoms of joint and muscle pain. I know that I'm prone to tiredness as my miles start to build, but for every day that I don't run ( on schedule) I feel a tinge of impending doom and a voice that says: If you don't trust and follow the plan you will never make it to the finish". I will finish and I WANT to finish STRONG. It's early, yes, but that's an excuse. Besides, so much of the training is grueling that I can't just let things slide when I don't feel "on form". 

How can I listen to my body without making excuses in my training?

*Cough, Cough, Cough*

Please advise.