To work out harder and longer, you need to build your endurance. However, endurance means a lot of different things to different people. To runners, it means being able to run for a long time, covering many miles. To weightlifters, it might mean being able to lift heavyweight for a lot of reps. Or to athletes, it can mean getting through an entire practice or game without needing to rest.
No matter the setting, endurance refers to your ability to carry out any given physical task for an extended time. Two parts of your body — your heart and your muscles — both contribute to giving you endurance. While they are both critical in helping your body go the distance, knowing the difference between cardio and muscular endurance is key to reaching your fitness goals.