The heart at Rest.
My resting heart rate this morning was measured at 39 bpm. This is maintained without any endurance or prolonged stressful activities too. You don't have to do endless steady-starte cardio workouts to improve cardiovascular health.
Why is this relevant? Well your resting heart rate is one indication of aerobic fitness. Exercise makes your heart stronger and more efficient, each contraction pumps relatively more blood and needs fewer beats per minute to do its work.
Consider perhaps the most important muscle in the body, not just the muscles you can see in the mirror.