The most important fitness variable for every triathlon distance is endurance. You might have top-notch form, high-end speed and high-value equipment; but without the proper endurance foundation you can count on an overall slow (a pace well below your capability) race or one that bleeds speed right to the end.
Ask yourself this: "How much endurance do I need?"
The answer: Your longest workout time should be 50- to 100-percent of your predicted race time. The shorter the race the closer you will be to the 100-percent time in a single workout. For well-trained or experienced triathletes, the longest workout preceding a sprint-distance event will be more than 100-percent.
For beginner and intermediate triathletes, many racers can complete a sprint distance triathlon between 1:00 and 1:30. For those athletes the longest bike ride in the training schedule will be between 1:00 and 2:00, depending on the athlete and how many weeks of training precede the race.
As race distance gets longer, for racers doing IRONMAN-distance events, my rule of thumb number is spread over one to three days. For example, in my popular 13 Weeks to a 13-Hour IRONMAN Plan, the biggest weekend combination adds up to 9.5 hours over two days. Assuming a 13-hour finish, the big endurance weekend is 73-percent of predicted race finish time.
Takeaway: Build overall race endurance.