A cursory examination of the door confirms your fears: it is far too thick to be sawn open. However, a closer examination of the air vent reveals that it was once a much larger window that has recently been partially boarded over. Confident that you can gain entry this way, you set about sawing through the slats.
It is exhausting work, and before you make a hole large enough to climb through, you lose 1 ENDURANCE point due to fatigue.