05/06/2014 04:53 pm ET Updated Jul 06, 2014

Don't Let Your 'Can't Dos' Get You Down

Ghislain & Marie David de Lossy via Getty Images

This past weekend, I completed a 10-mile race in my hometown. I was one of 40,000 runners, each with their own story, I'm sure. The fact that I ran the race is really not that Earth-shattering; a lot of people run 10 miles, or more.

What made this particular race so important for me was running it after hearing: "You can't do that."

I hate when people tell me what I can and can't do. People tend to underestimate me all the time. Maybe it's because I'm small or maybe it's because I'm a little older; most likely, it's because they have no idea the level of determination I have!

I always root for the underdog, because for most of my life, I've been the underdog. For as long as I can remember, people told me what I couldn't do. It started with my parents (where it starts for most people), but it didn't end with them. There was always someone in my life deciding what they thought I was capable of doing... how rude!

My parents told me I couldn't go to college; I went anyway. My mother told me I couldn't join the Army; I did anyway. My teachers told me I would never be successful if I didn't "calm down and get serious," but I'm 42, still silly and doing alright for myself!

Recently, I had "the talk" with my doctor. Not that talk, I had that talk with my dad when I was 12, and I'm still scarred for life by it! I had the "you can't do that" talk with my doctor.

Running my first full marathon was my 40th birthday present to myself. My doctor told me I shouldn't do it. Running is too hard on your body, it's bad for your joints, etc. By the way, I love my doctor, but he's extremely overweight. I love when unhealthy people tell me how unhealthy my lifestyle is!

I ran that first marathon and went on to run another. Over time, the message that I "shouldn't run" turned into a very serious "you can't run."

I'm over 40 and have severe arthritis in my back, hips and knees. I have a fractured knee that never set correctly and a minor heart condition. When I found out about the arthritis, the doctor told me I could no longer run or practice martial arts; he clearly didn't know me well.

Since I've turned 40, I've run two full marathons, eight half marathons, five 10-milers and dozens of other shorter races. I also got my black belt in Tae Kwon Do.

I'm not sharing this to brag, I'm sharing this in the hopes that it will inspire others to rethink their "can't dos." Now, to be clear, I'm not suggesting that you put yourself in unnecessary physical danger, but I am challenging you to decide for yourself what you are able to accomplish; I'm challenging you to not let others decide for you!

I get a certain sense of accomplishment when I reach a goal that others thought I couldn't reach. I enjoy the look of awe on their faces, that moment when they shake their heads and walk away perplexed. At that moment, being the underdog feels pretty awesome!

Many years ago, someone asked me, "Is there anything you can't do?" I responded that there are many things that I can't do, but I choose to focus on the things I can do.

This weekend I chose to focus on running 10 miles, at 40, with arthritis, a bum knee and a heart condition. Oh, and by the way, I ran that 10-miles with a kidney stone. Take that naysayers!