With the global population ageing, maintaining cognitive and physical independence is a major societal challenge. Regular physical activity and good cardiovascular fitness slow functional decline and help prevent age-related conditions. However, promoting sustainable behavioral change requires personalized, scalable interventions, which are often limited by the costs of traditional approaches. This project presents a digital coaching model based on a Large Language Model (LLM), integrated with physiological data from smartwatches. The system combines the Health Action Process Approach (HAPA) to support intention formation and action, and Kolb’s experiential learning cycle to foster reflection and learning. Its goal is to provide personalized motivational guidance to support the adoption and long-term maintenance of physical activity, promoting active health and healthy aging.