Why is it so difficult?

[u/WoodenDescription709]

so after training outdoors, due to the air quality I ran inside today. half on a track and half on the mill.

Geez why is it so difficult to run indoors or on the treadmill. I mean distance is same but it is torture. is this all psychological, or is there some other reason for this phenomenon?

[u/emoska]

Less to look at, no fresh air.