YSK cardio fitness is invisible Why YSK: A “fit”, muscular person might struggle to run 3 miles while an ordinary or "unfit" person might be able to run a marathon in under 4 hours. Cardio fitness, just like bodybuilding, takes years to develop, but unlike muscle development, cardio strength is internal and invisible. Endurance strength can vary tremendously from person-to-person. Based on heart rate, one person's walking pace might be the same effort as someone else running continuous 7-minute miles (4.3min/km). Never compare your cardio fitness to someone else's because everyone's athletic background and genetics are different.