Running, swimming, and biking have grown in popularity over the last several years, especially among adults who enjoy this avenue of competition now that their high school sports years are over. One thing these sports have in common is that they each require great endurance. To perform well in your marathon or triathlon, be sure to include endurance exercises in your workouts, so you can handle the long haul.
The main component of building endurance through endurance exercises is giving your body time to adapt. It is important to build up slowly. If you try to go from your regular 2 mile run straight to 10 miles, you will find yourself hobbling home with burning lungs and shin splints halfway through your workout. On the other hand, if you increase distance and time gradually, you will be amazed at how your endurance has improved. There are many couch potatoes who have become marathon runners by starting out with walking, then running 1 minute at a time with plenty of walking in between, and increasing slowly until they can do several miles without stopping.
Another endurance exercise that many athletes find helpful is interval training. Doing short segments of exercise at a higher intensity level with lower intensity exercise in between helps your heart and lungs to slowly adjust to the higher levels of intensity until you can perform at that level consistently.
As many endurance athletes will tell you, rest is a crucial part of building endurance. After you have pushed your heart, lungs, and muscles hard, you need to give them time to recover, so they can perform even better for you the next time. So schedule your rest days in and follow them as strictly as you do your workouts.