Transitioning to a plant-based diet while maintaining fitness requires careful planning to ensure adequate nutrient intake. Begin by incorporating a variety of vegetables, fruits, whole grains, legumes, nuts, and seeds into your meals. Focus on obtaining sufficient protein through sources like lentils, chickpeas, and tofu. It's crucial to monitor nutrients typically found in animal products, such as B12, iron, and omega-3 fatty acids, and consider supplements if needed. Additionally, maintain an exercise routine that includes both endurance and strength training to support overall health and fitness goals.