It’s not cortisol and probably not blood sugar and it sure as he’ll isn’t air density. It’s that after eight hours of not moving, muscles and connective tissue tend to stiffen up.
BTW, most world-class runners in Kenya tend to do their main run of the day at 6 am, while it’s still fairly cool. But they also tend to start off their runs at a ridiculously easy pace for a while before cranking it up.
Back when I was a gym rat, morning workouts were hell. I never felt as if I were even 1/4 as efficient and “with it” as I was for afternoon workouts. Maybe you’re just an owl? 
Okay, thanks. I’ll try google. I don’t jump out of bed and go walking. I’m an early riser so I’m up for a couple of hours before I walk, but still prefer mornings. Aside from beating the heat, I enjoy knowing that my exercise is done for the day.