Exercise is movement for the sake of movement, because science says, movement in certain ways was healthy.

In other words: run to lose weight, because that's healthy. Lift weights to build muscle, because that's healthy. Move exactly along this line for this amount of repetitions to maximise the amount of muscle you build. Rest for exactly this amount of time. You have no reason to take the stairs other than it being healthy. And on it goes.

Fun is not part of this equation so far, only conscientiousness.