Your heart and lungs don't really care what kind of workout you do. Running, swimming, bicycling or other continuous exercise can get your heart rate up and improve your cardiovascular fitness. The advantage for a runner of using other cardiovascular exercises is that it stimulates activity without stress on the legs and body from the impact of running.
Delivering Oxygen
Cardiovascular fitness is the ability of the lungs and heart to deliver oxygen to the muscles to create energy. Exercise increases the body's ability to generate this energy by increasing the oxygen supply, based on the intake of air from the lungs and the circulation through the bloodstream by the heart. The more oxygen the blood can deliver, the more energy the muscles can create.
Alternate Exercises
Bicycling, swimming, aerobic drills and even sports such as tennis and soccer can keep your cardiovascular training up while reducing stress on running muscles and providing a welcome break from pounding a path. A brisk bicycle ride after a hard day of running can provide the same cardiovascular workout as another run.
Improved Running
Cross-training may actually improve running performance. A study by the National Institutes of Health found that runners who added swimming to their workouts improved running performance while a comparison group which continued to only run did not improve. Exercises such as spin training on a stationary bicycle will increase your aerobic base for running.
Longer Workouts
You increase your cardiovascular fitness by exercising longer. Varying your workouts makes it easier to do this. If you're running 20 to 30 miles a week, it may be difficult to increase another 10 miles. However, you could easily add a 10-mile bike ride or an hour's swimming to get the same cardio effect. If your average run is an hour, you can increase cardio training by riding a bike for two hours.
