Introduction: Managing obesity and lipid levels is essential for the prevention of chronic diseases. Despite clinical guidelines emphasizing lifestyle interventions, the long-term sex- and age-specific trends in walking, obesity and triglyceride in South Korea insufficiently understood. Methods: This study analyzed national data to assess trends in walking practice, hypertriglyceridemia (hyperTG), and obesity among Korean adults aged 40-69 years from 2005 to 2023. Data were drawn from the Korea National Health and Nutrition Examination Survey (KNHANES), including 52,741 individuals for walking, 42,181 for hyperTG, and 54,079 for obesity. Joinpoint regression was used to identify trend changes and calculate annual percent changes (APCs) with 95% confidence intervals (CIs). Results: Results show that walking practice declined in men aged 40-49 years between 2005 and 2011 (APC: -10.2%), while women aged 50-69 years showed significant increases after 2012. HyperTG prevalence in men aged 40-49 years rose until 2020 (APC: 1.5%), but declined significantly in both sexes aged 50-69 years after 2011. Obesity steadily increased in men aged 40-59 years (APCs: 1.7% and 2.4%, respectively), whereas women aged 50-69 years experienced consistent declines (APCs: -1.9% and -1.5%). Conclusion: While walking practice has improved among older women, middle-aged men - especially those aged 40-49 years - face worsening trends in walking, obesity, and hyperTG. These research findings highlight the need for public health strategies tailored to the age and sex of middle-aged adults.