Racing at it's highest level is a physically demanding sport. 500 miles with a 120 degree track temperature can be brutal. A regular exercise program will improve your stamina and help you stay sharp over the course of a long hot race.
Also, a slim and toned driver will have an advantage over one that's heavier. In racing every pound counts and that includes the driver as well as the race car.
Get A Good Education
In NASCAR sponsors are the true key to success. You need every possible advantage in order to represent the sponsors well. A good education gives you the ability to speak well in front of the camera.
A racer represents his sponsor everywhere he goes. If you want a quality ride then you need the sponsors' money. Before they will write a check the sponsor needs to believe that you will represent them well.
In the early days of NASCAR you could drop out of school and be successful, with today's high-tech race cars and the ever increasing business side of the sport a high school education is the bare minimum. 1992 Winston Cup Champ Alan Kulwicki was the first ever to have a college degree, now it's becoming more and more common as drivers are realizing the importance of a good education.
Go For It!
Getting all the way to Sprint Cup is hard work. If you want to do it there is no "little bit." You've got to give it your all, all the time. If you make it you can be a legend, but if you don't make it you'll still have a bunch of fun and learn a lot along the way.
Good luck! And don't forget me when you become rich and famous.