Over the years, I've interviewed dozens of extremely successful people from a variety of fields. One of my favorite questions is simple: "What got you here?"

Their answers reveal a simple truth about success -- and the people who achieve success.

If you feel you're on the downside of advantage, it's tempting to rationalize that incredibly successful people were blessed with some advantage. Intelligence. Talent. Education. Connections. A special something.  

But that never turns out to be true. Almost to a person, highly successful people consider themselves to be average in almost all things. (Or even below average.)

Their only "advantage" was hard work.

Mental toughness. Determination. Willpower. Perseverance. Whatever word you use, the ability to overcome roadblocks -- to push through hesitation and discomfort and even just boredom, and keep working toward a long-term goal -- is what allows "ordinary" people to accomplish extraordinary things.

Take me. I'm decidedly average. But I like to work out, and since gyms are closed I built an outdoor pull-up bar. I normally do 50 pull-ups and 50 chin-ups as part of a back/biceps workout. 

But I've gotten bored with home workouts -- who isn't by now -- and as I was finishing my chin-ups the other day I decided to keep doing pull-ups until our daughter came back from a walk. Since I had no idea how long she would be gone, the uncertainty added a little spice: Not knowing when you'll get to stop makes staying the course even harder.

In time, I got to 200 pull-ups. Then 250. She still hadn't returned. Where the heck was she? 

Then I thought, "Instead of stopping when she comes back ... what if I do 1,000 pull-ups?" Stupid idea, but one that got stuck in my head.

So that's what I did.

Oh, it took time. A lot of time. Three-plus hours. (Do the math and, yep, my reps per set were really low by the end.)

Even though I felt "done" at 100 reps. And at 250 reps. And definitely at 500, and then at 800 reps.

That's the 40 Percent Rule, a concept popularized by Dave Goggins through Jesse Itzler's book Living With a Seal.  The premise is simple: When your mind tells you that you're exhausted, fried, totally tapped out, you're really only 40 percent done. You still have 60 percent left in your tank. 

In short, you always -- always --​ have more in you than you think. When you're doing something difficult and think you need to stop, you have more in you.

When you're trying to overcome a bad habit and are ready to give up, you have more in you.

When you're about to give up on yourself, because whatever you're trying to do just seems too hard, you have more in you.

Why? Because most of our limits are self-imposed. Over time, we've set those limits for ourselves.

How long we'll stick with a challenge before giving up and moving on. How long we'll stare at a whiteboard, trying to think of a way past a problem, before giving up and moving on.

How many calls we'll make. Emails we'll send. Proposals we'll create. Follow-ups we'll make. 

Those limits only seem real because habit has created them.

But they aren't real.

Think of a time when fear helped you push past what you thought was a barrier. Think about a time when a huge incentive helped you push past what you thought was a barrier.

Then, you could do more.

Because it turns out your limit was only 40 percent of what you were truly capable of achieving.  

The next time you think you've reached your cold-call limit, make one more. The next time you think you've reached your employee development meeting limit, conduct one more. The next time you think you've reached your quality double-check limit, check one more order. 

Challenge yourself to see if you can endure just a little more.

You'll find out you can.

And you'll realize that your limits are self-imposed.

And that you can accomplish a lot more than you once thought possible.