I wanted to jog beside the mirrors in the gym to evaluate my running style.
There were a lot of mirrors.
I could even see the 45 degree back view of myself.
...Gosh, my ass is huge.
...My legs are so fat.
...Eee... this is how I look when I run?

Too many thoughts in my head that the 1km was too exhausting.
I changed my treadmill to another one which has a nice outside view from the building.
I ran 2km without much effort.

See the difference?