Training for and participating in a triathlon works all of the major muscles in the body and provides several fitness benefits. All three disciplines provide aerobic benefits for the cardiovascular system.
Swimming is a great upper body workout, and it provides a cool, comfortable environment for workouts. Cycling can be done indoors or outdoors, and can improve balance, a skill that's increasingly valuable as we age. Running is a weight-bearing exercise, which helps maintain bone density. It's also the most convenient sport to do while traveling.