A tired 60-minute run

I ran for 60 minutes today (3.2 miles), but I was tired during most of the run. I felt fine during the first mile, but became tired as the run/walk progressed. My wakeup heart rate was 57, indicating my attempts to get more sleep are paying off. However, my body hasn't responded to the sleep as fast as my heart rate. It was a beautiful day with temperature in the mid 60s during my run. I wore a long sleeved t-shirt but would have been better off with a short sleeved shirt.