Runners and triathletes are often taught to obey the “hard-easy rule” in training. This rule stipulates that run workouts should either be very hard (say, 5 x 1000m at 5K race pace with 2:00 jog recoveries) or very easy (say, six miles at a pace that allows you to hold a normal conversation without getting winded). It’s true that this rule is helpful to the many runners and triathletes who, without it, fall into the trap of monotonous gray-zone training, where they feel compelled to make every run count, so they never go easy—yet precisely because they never go easy, they are also never able to go very hard.
But there is a place for moderately hard workouts, and the marathon-pace run is a good one. Warm up with one mile of easy jogging and then run anywhere from four to 12 miles (depending on where you are in the training process) at your ideal marathon pace. Doing this workout on a treadmill enables you to lock right on to that pace and stay there.