Runners who only run are prone to injury.
Cross training and weight training will make you a stronger and healthier runner, says TriEndurance.com multisport coach Kris Swarthout. Low and nonimpact sports like biking and swimming will help build supporting muscles used in running, while also giving your primary running muscles a rest.
The Exception The surest way to run better is to run. So if your time is limited, devote most of it to running.