I'm a young runner with big plans for my future.  I don't think anyone should ever limit themselves as I have yet to limit myself.  Think big and train big and your day will come.  The key to running is patience.  Most people don't have it and give up, all you have to do is outwork and outlast them.  Every day I look to improve myself.  Sometimes that means killing a workout, sometimes that means taking a much needed recovery day, and sometimes it falls in between.  What it all comes down to for me is the innate drive to compete.