fitness/health also generally improves mental performance (maybe hardly noticeably) and energy levels - which I personally find really important in uni when you have to actually get yourself up to go rather than know that your teacher will give you detention or something if you don't (like at school). Also if you don't have many contact hours, getting up in the morning to study is difficult if you never exercise, because exercise reduces fatigue/increases energy.
Also, people tend to put on weight at uni because there is greater availability of unhealthy/take away food (particularly if you go to uni in the city), and generally more alcohol consumption. So starting a weight loss/fitness regime now and keeping it when you start uni is great.
Also you probably had to do some sport at school (interschool etc) at least up until year 11, but at uni there is no such thing so doing some exercise by yourself or joining a sports club is the alternative if you want to stay healthy.
Finally, if you find uni difficult (the transition from school to uni is often hard, especially for people from high discipline schools) at the start, being fit/healthy is going to help you get through the tough times much better than being unhealthy would, as you will have the extra energy, focus and concentration required to 'get used to' a uni lifestyle.
Hence, exercising before and at uni is important and useful!