To improve fitness workouts must include:

Specificity – specific exercises and activities to improve an area of fitness..

Training – specific exercises and activities to improve a skill or type of health fitness

Overload – push a muscle beyond what it can normally perform. 

Challenge the body to work harder

Progression – gradual increase in overload

Frequency – working the body often helps increase your strength and endurance.

An effective workout has FITT:

Frequency, Intensity, Time, and Type