The metaverse, building on recent advancements in Virtual Reality (VR) technology, will be a significant driver of social change. Impacting how people live their lives and fostering new applications across fields such as entertainment, fitness, and rehabilitation, these technological advancements facilitate higher engagement of users through immersive experiences. In the short-term, this helps users to overcome barriers to physical activity and it promotes lifelong retention of physical and cognitive skills, a fundamental part of healthy ageing. To fulfil this potential, VR technology needs to be able to empower individuals regardless of age, capabilities, and prior VR experience. However, current fitness applications largely target younger users, often overlooking the needs and limitations of older or vulnerable groups. This work, the result of a transdisciplinary collaboration between therapy science, health psychology, and software engineering, introduces a user-centred approach to utilize VR exergaming for promoting physical activity, tailored to individual capabilities and preferences. Our inclusive approach incorporates a comprehensive user model based on static and dynamic ability attributes. These are assessed through questionnaires, sensor data, and a platform for gamified VR fitness tests, such as the presented prototype for reach and mobility. This model allows for personalized recommendations of VR exergames. We showcase several adaptive exergame prototypes that can dynamically adjust challenges based on real-time sensor data, such as heart rate. Addressing inequality in current VR fitness experiences, this work contributes to the broader understanding of VR exergaming to facilitate the development of VR interventions for promoting physical activity among older or vulnerable communities.