You must know that the PE exam has become more and more important in China. It makes up 30 points of the whole points of the high school entrance exam in many provinces, But lots of students don't care enough about sports, So I'd say: Don't wait until you enter Grade Nine or until just before the exam! Start now!
In middle schools in America, teenagers have to run a mile if they want to pass the PE test. They are timed (计时), and many students compare their times with others It might be difficult for students who haven't got enough exercise to run the complete mile. They have to finish by walking! Students who are out of shape are out of breath (喘不上气) after running! They get poor grades and some of them even have to take the class over again!
For the 7th and 8th grade students, I think you still have plenty of time to get interested in sports. There are many different sports to try. As you know, besides running, the PE exam will include football and basketball in the coming year. Make sure you can find some sports that interest you!
There are many advantages to get fit! You will feel better and look better. You won't have to rush to get ready for your PE exams when you become a 9th grade student. The most important is that you won 't be trying to catch your breath while other students go quickly past you in physical situation and life!