To Run or Not to Run?

I keep reading that you almost never regret going for a run, but you almost always regret skipping a run.  I wish it were that easy.  As I wrote about here, I'm pretty sure being too stubborn to skip runs when I was injured is what derailed my marathon training last cycle.

Today, I woke up to a slightly sore right knee (the one that usually gives me trouble).  I had a PT appointment scheduled for this morning and was planning to do an easy three mile run afterwards.  After working my legs (especially the right one) pretty hard at PT for an hour, my knee still felt off, so I decided to skip the run and add a few pushups, tricep dips, and crunches instead.  I'm hoping it was the right decision.

This training cycle I'm trying to be better about listening to my body instead of blindly sticking to the schedule, but I'm having a hard time with it.  The only reason I felt comfortable skipping the three miler today is that I can fit it in on Thursday instead of spinning, so my weekly mileage won't be affected.

So, fellow runners, let me know -- how do you decide when a run is going to do more harm than good?