If at first you don't succeed, abort the run

I started on my 5-mile run but could tell I was tired. I didn't have the "spark" of energy that I had on Saturday and yesterday. So I aborted the run and did 3.5 slow miles. Depending on how I feel tomorrow, I may skip tomorrow's run. I'll be doing my 5K simulation this week on Friday instead of Saturday, and I want to be well rested for that workout. My wakeup HR was 51. I was out two hours later than yesterday, and the temperature was in the low 80s (F) during my run.