Generate a full detail guide on the mental health benefits of physical activity. The guide should be motivating and accessible. It must include sections on: 1. The Science of How Movement Affects Your Brain. 2. How to Find a Form of Movement You Genuinely Enjoy. 3. A Beginner's Guide to Starting a Routine Without Overwhelm. 4. Tips for Overcoming Common Barriers like Lack of Motivation. Please include a disclaimer to consult a doctor before beginning any new exercise program.