"Endurance Cross-Training" is very important for runners to incorporate into their programs. What's best for you depends on your needs, goals, experience, and preferences- just as it is with any other kind of training! My favorite form of Endurance Cross-Training is swimming. With swimming, the arms produce far more force than the legs. Swimming does give the cardiovascular, metabolic, and endocrine systems a proper workout without stressing the tissues of the legs. It is a perfectly good way for runners to get an active-recovery workout, especially the day after a long run. It is also great for individuals who have serious injuries that make it impossible to perform leg-reliant activities! Other types of Endurance Cross-Training that people enjoy include bicycling, inline skating, and cross-country skiing. There are many benefits by incorporating 1-2 days of this type of training during the week!