"We all have limits. Almost no one reaches theirs. You definitely haven't."
You could swim faster if a shark was after you. You could run faster if your child was in danger. You could work harder if the payoff was truly exceptional.
What you think you can do is always--always--less than what you can do if you really, really try. You always have a little more in you. Find your true limits and you may find that success is limitless.