Number 2 when running

When I run on the treadmill at an incline I always feel like I need to stop and do number 2. If I am not on an incline I don't have this feeling.
I hate stopping in the middle - or when I have just a few minutes left on the timer. The idea of putting sweat soaked shorts to continue running is gross.
I always have number 1 before I start exerciding. But only number 2 during a run - never while on a bike or an ecliptical machine.
Why does this only happen when running? How do I prevent this?