I know we have all said "I'm not feeling so well" several times in our lives. Though I am living a healthy lifestyle, I am still at risk.
I know there are different opinions on working out while ill; such as above/below (the neck, where if your sick above your neck light work outs are fine, below you should rest), however I believe in listening to your body.
Having two young children, it is hard to have the energy on a good day so when I am full blown sick I refuse to work out. If I have a small sinus problem I will keep on trucking along.
If you do feel it is best to rest while sick, do not beat yourself up! Not only are you punishing yourself over something out of your control, you might be stressing yourself out more and that is not how you get healthy.
What you need to do is; Rest; drink tons of water; rest some more; and build your energy back to where it needs to be. I know it is hard, but try not to get discouraged about losing everything you trained for. You didn't lose it! True when you are healthy again and ready to pound the pavement, you might not have the same mileage as you did before the sickness. Though two or three workouts later you will be right back on track. Now compare those three (workout) days compared to a month of training, it doesn't seem so bad.
Perfect example. I was very ill last week. I could not move off the couch so I decided it would be best if I rested. Now I was running anywhere from 4 to 6 miles a day. After two workouts, I ran 4.5 miles tonight; I would have kept going but they kicked us out!
So, just listen to your body. If it is telling you that you can't do it, I don't. Don't get discourage and quite, turn it positive and say "Once I feel better, I will do better than before!"