Most solid training plans include a weekly speed workout, often in the form of intervals. But the length of those intervals usually depends on your race distance, training goals, and fitness level.

Learn more about this metric and the best training sessions to add to your schedule to boost your cardio fitness.

Good news: starting your running journey doesn't have to require diving in head-first and sprinting for 30 minutes straight. If you’ve never been much of a runner (or you’re starting to get back into ...