Whether you are a novice runner or are already accustomed to running for fun or fitness, you can improve your speed, endurance and enjoyment by increasing your cardio respiratory endurance. Though you may associate tiredness with aching legs, the limiting factor to how fast or far you run is more likely to be your cardiovascular system. The stronger your cardio system, the more oxygen you get into your blood stream. This helps prevent the buildup of lactic acid, which causes muscle soreness while running. If you have an injury, or any concerns about beginning or increasing your running program, see a doctor.
Cardio respiratory endurance falls into two categories. The first is aerobic endurance, where your body is working at a level where the demands for oxygen and fuel can be met by the body's oxygen intake. And anaerobic endurance, where the body is working so hard that it needs more oxygen and fuel than is being supplied and you go into oxygen debt. The key to building your cardio respiratory endurance is to increase the speed and/or distance you can run without going into oxygen debt.
If your current mile time is more than 8 minutes, 30 seconds for a man, or 10 minutes, 30 seconds for a woman, begin with a run-walk program for four weeks; gradually increasing the amount of running each week. You should then move to running steadily for a prescribed distance, working on increasing your speed gradually. This will develop a solid base of cardio endurance.
To rapidly improve your cardio respiratory endurance, you need to do interval training. This can consist of hill training, which challenges your body more than running on flat surfaces, or speed work. If you are new to speed work, the book "Morning Cardio Workouts" has a simple 20-minute program that will help you ease into it. Begin with a four-minute jog, then increase to a challenging pace for two minutes, jog for three, then sprint for 30 seconds. Repeat the sprint/jog section three times, ending with a cool down to return to normal breathing.
You don't have to just run to increase your running endurance -- cross-training can be a valuable tool to build your cardio system, avoid boredom and prevent injury. Use a variety of high-intensity exercises to enhance your cardio respiratory endurance. Intervals of intense cycling, inline skating or step workouts can boost your endurance; as can sports such as basketball and tennis, which require quick bursts of high-energy movement.