Endurance competition preparation involves complex psychological, logistical, and sociocultural factors. This study investigated the pre-race nutrition practices of female endurance athletes, by applying a newly developed extended Theory of Planned Behaviour, including behavioural execution to the framework to capture real-world influences (ETPB-X). Using a convergent mixed-methods design, 27 female triathletes competing at the 2024 IRONMAN World Championships completed questionnaires, 48-h food diaries, and semi-structured interviews before and after the race. Quantitative data (n = 23) were analysed for energy and macronutrient intake, whereas qualitative data were thematically coded using the conceptual ETPB-X framework, incorporating attitudes, subjective norms, perceived behavioural control (PBC), utilitarian drivers, and behavioural execution. Only 26% (6/23) of athletes achieved carbohydrate loading guidelines (8-12 g·kg-1 body mass (BM)·day-1), with an overall mean intake of 6.4 ± 2.1 g·kg-1 BM. Higher carbohydrate intake correlated with faster finish times (r = -0.50, p = 0.035), whereas fibre intake was positively associated with gastrointestinal (GI) symptom severity (r = 0.85, p