What have you done to test yourself physically? Are you an athlete? Have you ran any organized races or trained for any competitions?
If you have, then you will understand that you must start slow and build. Not many people can jump right into a competition and do well or avoid injury. Basic training is the same way. You need to be physically prepared before going into basic. They do not have time to give you the foundation of physical fitness. You will be expected to have a certain level of fitness. This is why they give you that preliminary PT test to see where you stand. Why not go basic training in the best shape you can to ensure your success. It will also make basic a little easier on you as well.