Running has taught me:
You have to get through the tough parts to get to the good. Sometimes when I take off for a run, the first mile is so challenging I can't imagine how I can possibly make it to the end. Each step is a battle mentally and physically. It's the longest mile. For whatever reason (stubbornness? determination?), I keep trekking along. One day I had a ten mile run on the schedule and I didn't 'hit my stride' until the 6th mile. That means for about an hour of running, I was hurting, both mentally and physically. But once I got my 'wind', I was feeling great. If I had stopped when it didn't feel good I never would have made it to the good.
I can't look at the total distance. I know it's there. Whether it's 4 miles, 13.1 or 26.2. If I think of how far I have to run, my mind begins to fill with doubt and negativity. I start hearing words like: Why are you doing this? You can't run that far! You're hurting, just stop. Instead, I have to focus on the mile I'm running. And I think of how far I've come. I have a 10 mile route that I run nearly every Saturday. I focus on getting 3 miles done. And instead of letting myself think about the 7 more I have to do, I tell myself 'just 2 more miles, you can do 2 more miles'. Once I'm at my 5 mile mark, I don't let myself think 'you are only halfway there', instead I think to myself 'you just ran 5 miles! Now get 3 more miles done'...if you are doing the math with me we are at 8 miles. Not once do I let myself think "you have to run 10 miles or you have 7 more to go". If I start thinking that way, my mind will try to overpower my heart and tell me to give up. I cannot focus on how far I have to go. I have to break my run up into smaller miles and feel good about the miles that are behind me, not in front of me. 
You have a strength in you that is more than muscle. It's a strength that you find deep down in the core of what makes you you. I didn't have the words to describe it until I found a poster on pinterest that described it perfectly: Beast Mode. If you think you don't have it, you are wrong. It's in you. You may have seen it when you went "Mama Bear" and had to stand up for your kids. Trust me, it's there. You don't always need to use it. But when you do, it's what will help you get through something that you thought was impossible. 
You cannot compare, judge or worry how you match up to someone else. Focus on yourself. I'm never the fastest, thinnest, or strongest runner out there. I have learned that someone out there is always judging you: for exercising too much, for exercising too little, for being a stay at home mom, for being a working mom. The list goes on and on. The only person I can worry about is me. When I run, whether it's on the treadmill at the gym, a route in my neighborhood, a race with thousands of people, I run for me. Everyone else is running their own race. Let them. And focus on you.
Here's what I truly love about running. Every single lesson it has taught me, can be applied to my life. My running coach has a saying: "Run with Joy". Not only do I want to run with joy, I want to live with joy. I hope you do too.
