Worrying does make things worse. Sucking is how you're supposed to be your first time out. Embrace the sucking. Accept that you're new at this, that sex is a skill that is never masterred, and one that you will improve as you go through life. Accept that your girl will either accept that you're a novice or she won't. If she does you have a great relationship. If she doesn't accept your staying power or skills her standards aren't realistic and more a reflection of her lack of experience than yours.
The PC muscle is short hand for the Pubococcygeus muscle. If you try to sound that sucker out it gets to be clear why it's allways abreviated. Depending on context it is either treated as a single muscle or a muscle group. It is responsible for most everything that moves in your pelvic region. Kegels strengthen the Cremaster Muscle, a part of the PC group that performs a number of functions and is linked to both erection and urinary function.
Kegel excercise improves most aspects of your basic sexual performance. This improvement is over time. unless you're planning on holding off on having sex with your girl for a decade you're not likely going to see much improvement in staying power. However training your PC muscles and preventing muscle erosion through regular kegel excercise is important to establish healthy sexual function longer in your life.