Specificity Principle
The principle of specificity states that training adaptations are specific to the system worked. For example, to improve running performance, one must run rather than swim. It also means that within a given sport, the type of training needs to be geared for the type of racing to be done or performance gains desired.
It is true that endurance training results in a certain amount of central adaptations—that is, general adaptations to the central respiratory and cardiovascular systems. However, these improvements will only go so far in improving your sport specific performance. You also need the peripheral adaptations that occur in muscle groups used for a particular activity. To return to our example of the athlete that swims in preparation for a marathon, that athlete would lack the peripheral adaptations that would occur in the legs with run-specific training.
For multisport athletes, the principle of specificity means that you must train in each of the disciplines used during your races. It also means that you need to gear the type of training you do for the particular distance you will be racing. A program designed for short-course triathlons looks different than a program designed for long-course triathlons, for example, just as a running program designed for a 5K looks different than one for an ultra-marathon. Finally, given that everyone is an individual and responds in their own ways to different types of training, it is important that you tailor training specifically to your individual needs and situation.