Running and fitness have different aims to lose weight. Running is to slim down and have a slim figure. Fitness and weight loss will add strength training, and finally get a curvy figure. Strength training helps to reduce muscle loss, increase muscle content, improve body metabolism and burn fat faster.
Running and fitness have different emphases on improving the inner body. Running can improve cardiopulmonary function and physical fitness, while fitness is mainly based on strength training, which mainly improves physical strength and muscle mass.
People who insist on running for a long time are more likely to gain weight, because the loss of muscle makes it easy for fat to accumulate again after returning to normal diet. People who exercise to lose weight have increased muscle content, improved body metabolism and lower probability of gaining weight.