Should I do sprints or long distance runs to improve my baseball endurance?

To initially improve endurance, running at a steady pace for an extended period of time is advised. When beginning a cardio program, start slowly and gradually progress the time and intensity of the workout. For example, begin with 20-30 minutes of low intensity cardio (roughly 65%-75% of your estimated heart rate maximum). In other words, on a scale of 1-10, the intensity of your cardio program should be around a 4-5. Once you have been able to sustain this endurance level, you can then begin to progressively increase your time for each bout of cardio activity. Once you've acheived a good baseline fitness level, you can also begin to incorporate interval training into your cardio routine. This will help your endurance because it will force your cardiovascular system to recover under more challenging conditions. To accomplish this, begin with higher intensity levels of 80% - 85% with a recovery of 60% - 75% intensity. You can then continue to progress into a more advanced type of cardio training where you implement very short bouts of intense activity at 86% - 90% of your maximum heart rate followed by a recovery period. There are also other benefits of interval training such as reducing boredom from steady-state activities and causing training overload of the heart and lungs leading to increased cardiorespiratory efficiency. Finally, interval training will increase an athlete's metabolic rate during recovery, basically causing an after-burn effect. In short, interval training is a highly efficient way to get a training response from minimal time.

If your asking this question. You are playing baseball. And to improve you baseball endurance you should do sprints.  Baseball is a game of quick burst of speed. If you are running after a ball in the field or running bases, it's stop and go with rest inbetween pitches. As a former pro baseball player this question is always asked and I agree with some of the best strength coaches in the game who have there players only doing sprints to improve endurance while playing baseball. Do lots of sprints. On a baseball field go to the warning track, start at the foul pole and sprint to the centerfield wall. The jog to the opposing foul pole, turn around and do it again. The more sprints the better.