The game adds extra mileage to your statistics when you move from one floor to another. After testing every staircase in Dante Manor, I discovered the one with the most miles per transition. The staircase is placed in the house's left wing, between the second and third floors. Changing floors at this point will quickly accumulate the mileage needed to unlock the "Marathon" and "Tour de Dante Manor" achievements.



Location of the staircase on the map



Entrance to the staircase from the second floor. It's located right next to the male-staff dorm, in the corridor connected with the lobby.



Staircase. When you enter it, go up and through the door.



Entrance to the staircase from the third floor. It's directly next to the grandfather's study. After exiting the staircase, come right back in. Continue until you've unlocked the desired achievement. Each transfer adds 80 miles, therefore it takes 13 transitions to cover about 1000 miles.

If you are still lost and in need of direction, please refer to the following video:
(flashing imagery warning!)