Among the few running experts who have explicitly addressed the matter of optimal training cycle duration is Jack Daniels. In Daniels’ Running Formula, he identified 24 weeks as the ideal training duration for every race distance and for runners of every level, with some exceptions. All of the training plans in the book, for races ranging in length from 1,500 meters to the marathon, were 24 weeks in length. Although Daniels did not explicitly explain why he considered the 24-week duration ideal, we can deduce from his overall explanation of his training system that it simply takes 24 weeks to work through all four phases of his periodization method, but no longer. It follows that if you have already completed the first part of this training process, or something resembling it when you set your sights on a specific race, then you don’t need a full 24 weeks to prepare for peak performance at a given race distance. For example, each of Daniels’s plans begins with a 6-week aerobic base-building phase. If you already have a solid aerobic base when you decide to train for a specific race, then presumably you can skip this phase and devote only 18 weeks or thereabouts to building toward peak fitness. Not only are 24 weeks enough time to cultivate peak fitness for any race, but they are also approximately the maximum amount of time a runner can train progressively without burning out. Reaching lifetime peak fitness takes years, and this multiyear process must be broken into individual cycles separated by brief periods of regeneration in which some fitness is intentionally lost. A runner who attempts to continue improving his or her fitness indefinitely, even from a low starting level, is likely to find that burnout (or injury) occurs after 24 weeks or so.