The "Cardio" skill involves sprinting, you can sprint by holding the
button. (you can do this on any character)
(if you stop sprinting DON'T WORRY the character is gaining back it's stamina).
Once you reach "Max Level" for "Cardio" you can specialize it by pressing the
d-pad then going back by the to the stats category by pressing
then going to "Cardio" and pressing
(Then the achievement will unlock).