Hmmm I just finished a 4.5 mile run that was hell......yet last week 10 miles was a breeze........I think it's just the way it goes.
Personally I find running in the morning particularly difficult, and the evenings are my best time!! Dont know if thats because I have fueled up all day? Possibly.
I ran a 10k race a couple of weeks ago, a distance that I would normally take for granted, but the heat of the sun killed me and although I was fast for me to 5k.......I was extremely slow for the second half. I find running in heat unbearable, another reason why the evenings are better for me I guess.
Anyway it appears there is no set answer to this dilemma.