1 flight of stairs.

Cardio builds very little muscle. Take it from the mouth of a quite competitive high school runner. Used to run about 30km per week (not high by many standards, exceedingly high by my current standards). Diet and lifestyle are more important for fitness. Endurance exercise is pretty bad for the heart, just look at the marathon runners. Ouch. The thing is cardio becomes addictive as most people have low intrinsic energy (due to diet, lack of sun, etc...) so when they release stress hormones through cardio they now feel a bit better. Not very sustainable in the long run. Look at our history as a species and nowhere do I see much cardio until the 20th century. Activity for sure, sign me up. Cardio as understood nowadays with endless running and crossfit? Only thing I might actually run away from