In 1962 the great New Zealand running coach Arthur Lydiard wrote an article titled "Marathons for Milers" for Sports Illustrated. In it he explained why he required all of his athletes, including the 800-meter runner Peter Snell, to run 20 miles every Sunday.
"In theory," he wrote, "I am trying to develop my runners until they are in a tireless state. In practice, this means I am trying to give them sufficient stamina to maintain their natural speed over whatever distance they are running. Stamina is the key to the whole thing, because you can take speed for granted. No? Look here. Everybody thinks a four-minute mile is terrific, but it is only four one-minute quarter miles. Practically any athlete can run one one-minute quarter, but few have the stamina to run four of them in a row. How do you give them the necessary stamina? By making them run and run and run some more."
More: The Medium Long Run: A Triathlete's Secret Weapon
Before Lydiard came around, only marathoners did 20-mile training runs. But after Peter Snell won a gold medal at 800 meters in the 1960 Olympics, the practice quickly spread throughout the sport. More than 50 years later, long runs remain a staple workout for elite runners specializing in all race distances from the mile to the marathon.
The principle of using long workouts to build stamina for short races applies to triathlon just as well as it does to running. If you're like most recreational triathletes, your longest swims, bike rides, and runs are not nearly as long when you're training for a sprint triathlon as they are when you're training for a longer event. But if you want to improve your performance in shorter triathlons, increasing the distance of your longer workouts is a good idea.
Triathletes tend to think of long workouts as serving the purpose of building the endurance that is needed simply to complete a long race. For example, you may do one or more 100-mile bike rides while training for an Ironman to ensure you're able to complete the Ironman's 112-mile bike leg. But what if you already have the endurance you need to complete a given race distance? In that case, long workouts may serve the slightly different purpose of helping you avoid slowing down toward the end of the race.