Running is a great stress reliever but it also puts a lot of stress on the body. Without a strong core, your gait is likely to be negatively affected. Cycling is a great non-weight bearing sport but sitting on a bike and pedaling doesn't translate to a strong core. If you love to swim,bike, run to be faster, you also have to train to be stronger. Get stronger before you go faster before you go longer. Every time you land your foot on the ground while running or when you take a pedal stroke, you are relying on your pelvis, abdominal muscles, hips and lower back to work properly as you move your body in a forward motion. If you are currently training for an event and find yourself neglecting core/hip/glute strength work in your cardio training plan, it's time to carve out 10-20 minutes, a few times per week, to build a strong core. Sure, training swimming, biking and running may make you a faster triathlete but putting in the miles won't not necessarily...
3x Author, Board Certified Sports Dietitian, Master of Science in Exercise Physiology, 2017 IM CHOO Amateur Female Champion, 19xIronman finisher including 6xIM World Championship finisher, Triathlon Coach, 30-year Vegetarian.Trimarnicoach.com