If you are struggling to run a 100 metres, then you are unfit. One way to rectify this would be to exercise. There are lots of courses in fitness available but if you can not follow a fitness course on your own, then a wise move would be to use a fitness trainer.
If you are not unfit and enjoy being active then you can help others. A personal trainer course will help you to learn how to motivate and boost others to lead the same healthy lifestyle that you do.