Cardiovascular fitness and muscular strength-endurance aren't the same thing

Good aerobic conditioning doesn't guarantee you'll last long in a squat

People often use the word 'endurance' as one blanket concept, but physiologically, cardiovascular fitness (how efficiently the heart and lungs deliver oxygen to muscles during sustained effort) and local muscular (strength) endurance in a specific muscle group are partly independent qualities, each requiring its own type of training.

A marathoner with an exceptionally efficient cardiovascular system can gas out holding a plank or in a high-rep barbell squat set, not from a general shortage of oxygen, but from local fatigue accumulating in specific muscle fibers, limited by their own metabolic and structural characteristics rather than overall heart function.

That's why improving long-distance running performance doesn't automatically carry over to doing many squat reps, and vice versa — a strength athlete with excellent local leg muscular endurance can be gasping for air within a couple minutes of easy jogging. For balanced fitness where both qualities matter, they need to be trained separately rather than relying on one carrying over to the other.