In retrospect, I'm happy to know that it was just a cold that was affecting my stamina and not something more serious. I took my usual rest day on Monday and, by Tuesday, it was clear that a run would do me more harm than good. However, I did cover five miles walking in the city that day. By Tuesday night I was officially exhausted. I spent much of Wednesday trying to rest and recover.
I'm still dealing with a full blown head and chest cold, so running is out of the question for now. I'm feeling a slight improvement today and I'm hoping that continues. I doubt I'll run tomorrow, so this may turn out to be my lowest mileage week in over a year. That's fine, since I run 5-6 days a week, it's probably good to take a long rest every once in a while. It took a bad cold to make me stop, but maybe all this rest will do me some good.