If lung cancer isn't detected until it's causing trouble breathing, has it usually already spread to other organs?

It depends.: It depends on a few things: 1) where it is in the lung, whether in the periphery or near the large airways; 2) what type of lung cancer it is (some are known to spread very early and quickly, others less so); 3) what the health of the person was beforehand (people with early copd, for example, may have breathing difficulty with lung cancer much earlier than someone without any other problems).

Not always: Symptoms like breathing difficulty may happen by blockage of the airways or blood vessels- this is more common with cancers located towards the center of the chest and is unrelated to cancer spread. At other times, cancer spread to both sides of the lung causes breathing difficulty, smoking related emphysema makes the breathing worse.