PHILADELPHIA — Andrew Hunter has the complexion, shaggy blond hair and blue braces that befit the most common of 16-year-olds, but the Loudoun Valley sophomore was running in the Penn Relays 3,000-meter championship of America on Friday because he has grasped what most teenagers can’t. He has fully accepted his mother, Joan Hunter, as his high school coach. He has put himself on a strict sleeping schedule ever since he started running last year. And his diet is prodigious, according to his mother, who watched him wake up Friday and proceed to eat a planned meal of fresh vegetables, hummus, ham slices and gluten-free granola.