To those who are overwhelmed by the prospect of what is required to reach these goals, my advice is this: Be better than you were the day before. That's the secret. That's what drove those athletes to greatness.
Weight does not measure human worth. It is not an indicator of character. Bathroom scales are not designed to weigh merit. The boy in the Nike ad may well be full to the brim with greatness -- but none of it has anything to do with running.