It takes a whole-body approach to be fit. That includes consistent cardiovascular activity (e.g., cycling, running), strength training (via weights or calisthenics), and flexibility/mobility/balance enhancements through regular stretching/yoga sessions. To achieve the best fitness results, combine these activities with proper nutrition intake and ample rest. By taking into account the oversimplified fact, "If you burn more calories than you take into your body, you will lose weight, and if you take in more than you're able to burn off, you'll gain weight." it is much easier to keep in mind what and how much you should be eating.