Assume that you take in 2,000 to 2,200 calories a day from food just to maintain your weight. 6k of running is roughly about 370 calories. Subtract that you only got less than 2,000 calories left! So to answer your question, you need to eat more (increase your calories intake) in order to gain weight. Do it in a healthy way though, like eating more carbohydrates (rice, noodles and bread......etc.). Keep doing your daily 6K run. Your體能training will also help you to build up muscle mass too, so it is good.
