Hello, I am a professional athlete. You said you want to improve your performance by one kilometer, but it's not enough to exercise your leg muscles. The best way is to get up early in the morning and go to school for a long run. How far you run depends on your personal situation. When I was in junior high school, I jogged 3000 to 5000 meters every morning, mainly for endurance. I recommend you to practice 400 meters running and short sprint. The most important thing is to keep practicing. This kind of thing is most afraid of giving up halfway.