Good for you for taking the stairs. This is a healthy life choice. Up to a point will improve a person’s fitness. But unless the exercise load is increased, your body will adapt to that level of stair climbing stress and you will not improve more. A gradual progression in training load is what separates “training” form “exercise”. While climbing 100 flights of stairs is a lot of exercise and will help you develop and maintain a good base level of fitness it will not be the ideal way to maximize your fitness gains.