This chapter presents a longitudinal case study conducted over the course of 1 year with a high-performance open-water marathon swimmer, exploring how genomic profiling was used to personalize and periodize out-of-water physical training. Expanding on content previously published in a scientific article, this chapter offers a month-by-month narrative from April 2023 to January 2024, integrating genetic data with training plans, technical reports, and records of conversations with the coach via WhatsApp. This practical and integrated approach highlights not only the athlete’s physiological evolution but also how genomic information can inform real-world training decisions. The chapter offers a unique perspective for coaches and sports scientists interested in precision training strategies applied to endurance sports.