Not sure if it's the "mornings" or simply your energy level. The reason I say this, is because my energy level fluctuates. Some days I can barely make it through my workout and others, I feel like I could run another 5 miles!
I see your local to me. As the weather gets better, you may find yourself enjoying the fresh early morning sun at Byrd Park!
That's what I do...they have a wonderful trail there!
I thought I was gonna die on my treadmill yesterday, but the week before I ran at the park, after run 5 miles, I felt as if I had seriously had at least another 3 miles left in me. Daily hormonal changes/energy levels will fluctuate. Always push yourself, BUT accept that you will not perform at peak performance every workout and remember to listen to your body!
Congrats on making it through your workout!