The core muscles reside in the area of the stomach and mid- and lower-back and I would include the hips, shoulders and neck. All of these muscles are connected and are used to help stabilize the body. As a triathlete, keeping all of these muscles strong will pay dividends in the later stages of a triathlon especially an Ironman distance race.
As endurance athletes we focus on the swim, bike and run countless hours in the week and find very little time or have the desire to hit the weight room. At a minimum we should take 10-15 minutes two to three times a week to conduct core workouts. It will decrease the risk of injury and help maintain form during the later stages of a workout.