Training

Training for a school cross country, for a sport or even just trying to get fit can be tiring. If you don't do a lot of exercise, you can overdo it. Even if you are used to doing a lot of exercise you can still push yourself too hard. Don't try to do it all at once. Getting fit and doing lots of exercise takes time. Yeah, that means doing it in small amounts, working up to your goal. 

Just keep in mind. it takes time.