the faster you move through space, the slower you experience time.
if one sent a friend into space on a trip for a few decades, and the friend was going the speed of light throughout the trips duration, when the friend returned back a few decades later, it would appear as if they havent aged a day. and physiologically they wouldnt have aged either.
since on a treadmill your not moving through space at any type of speed, and outside walking you are moving thrrough space at a measurable speed, then your body will experience "30 minutes" of cardio as a shorter amount of time if your outside walking, than if your inside on a treadmill.