Run Forrest, Run!

How many times do I tell myself, "I'm going to go run 5 miles today?"  I bet you can already guess what I tell myself..yes I always talk myself out of it.  But when I do run, it feels so great after!  Is there a limit to running? Can you be too tired to run?Obviously not.  So I ask, How far is too far to run?  How far have you run before?