Yes, it's really all mental. I know many times when I'm on the treadmill and wanting to quit I have to tell myself that my legs feel fine, my breathing is fine, so I can KEEP GOING!
I run outside. I am not up to 5 miles yet, but I do 4. I could not possibly do it on a treadmill it would be way too boring. I even have to switch up my run so I don't run the same every day becaues that gets boring to me also.
