Cardio, short for Cardiovascular Training, is used to keep your heart rate up in prolong sessions using little to no weight and constant repetitive movement.
You will get a large increase in endurance which can translate to adding more capacity for physically demanding work. Another benefit is speed if you practice short burst of full intensity cardio, also known as anearobic exercise.
The difficulty is building up the speed and endurance needed for cardio. Other forms of difficulty will form like being out of breath, or increasing minor injury's like shin splints or runners knee.