To Run, or Not to Run?

So I started my half-marathon training this week.  I've been alternating days of running with Insanity.  It's a lot, but I thought I could handle it.  Today is supposed to be my "long run" of 5 miles.  However, I noticed yesterday that I was worn out.  Getting my three miles in was tougher than normal and my legs felt even more like jelly than before.  So now I'm trying to figure out if I should go and do my run, or if my body is trying to tell me to take a break.  What would you do?