Did you know you should be increasing the intensity, frequency and/or duration (acute variables) of your exercise over time in order to improve your results? (Periodization).
You can use some activities to fulfill more than one of your basic exercise requirements. For example, in addition to increasing cardiorespiratory endurance, running builds muscular endurance in the legs, and swimming develops the arm, shoulder and chest muscles. If you select the proper activities, it is possible to fit parts of your muscular endurance workout into your cardiorespiratory workout and save time.