Slowly build your mileage when you run, or lengthen the time on your bike or your swim. Don’t overdo it, or you could injure yourself. Increase your mileage or time each week, and on the fourth week, scale back and give yourself a recovery week. A variety of cardio activities can also boost endurance. With winter approaching, you can mix up the usual activities and try snowshoeing or cross-country skiing. If you want to avoid the cold, then try the elliptical machine or rowing machine. If you have a time …