Delusions

At what point does hope and optimism turn into a delusion?  I took yesterday off so as not to strain my hamstrings, and thought that maybe the rest would give me more energy today. But instead of feeling invigorated and peppy, I felt even worse!  I managed to do the entire one and half mile loop without stopping, but only from dint of will.  What is wrong with me? Am I ever going to see an improvement? How can I feel so bad?