Can I train for a race on a treadmill?

from running.about.com

A friend of mine wants to run a half-marathon this spring. She's a mom to two young girls and works four days a week as a nurse practictioner, so she does most of her runs early in the morning, on a treadmill. She asked me recently, "Can I run a half-marathon if most of my training is on the treadmill?" Winter weather, limited daylight hours, and busy schedules put a lot runners in the same position. More