The reason I pushed the run back is because I felt kind of under the weather today. I have had a scratchy throat for the past few days, and today I felt lethargic and just kinda out of it. So I decided that a run might do more harm than good: it might push me over the edge of healthy into SICK.
One rule of thumb I've heard says that if your symptoms are in the head -- i.e. stuffy nose, sniffles, sinus congestion -- you can run. If symptoms are in the chest -- chest cough, chest congestion -- you should rest. But my symptoms were in my throat! Where does that count? Should I have run or not?
(The suggestion of Ian -- ever the engineer -- was that since my symptoms were halfway between my head and chest, I should simply run 1.5 miles instead of my planned 3. "It's simple," he said. Gotta love engineers.)