This is the first in a series of guest posts by Susan Alexander:
What I admire about Darryl is his openness to experimentation, learning, and change. For him, life is an ongoing quest to find the essence of what we all want to know: what works, what doesn't, what goes well together, what needs tweaking, and what makes it fun.
Like Darryl explores fitness, I explore change, across many contexts. My purpose is to figure out the same things about change that Darryl's figuring out about fitness (what works, what doesn't, etc.). We're like field scientists in our respective fields. Darryl's about what to do and how. I'm about why you'd want to in the first place, and how to get started and stick with it. He's the freight train. I'm the engine.
Why should you read this?
Your fitness and health are driven by what you think and do. This post is about the thinking part. Have you considered the thought process that fuels fitness and health? Have you ever wondered what's going on in Darryl's mind, or in the minds of people like him? That's what we'll explore here. 
Fitness, health and everything we do are driven by something called mindset. There are two kinds: the growth mindset and the fixed mindset. Which one you have will determine the course of your life, because it's the very thing that makes you think what you think and do what you do. It's the source of your whole approach to life. Mindset is what makes people be like Darryl, or not. Which begs the question ...
What's with Darryl?
Darryl has a growth mindset, which is based on this simple belief: that we can grow our intelligence, our skills, and ourselves throughout our lives. It's the opposite of the fixed mindset, or this belief: that who we are now is the way we’re always going to be (in other words, we can’t really change much). Darryl and I have never met, so how can I know his mindset? All the clues are on his site, practically popping out of the screen. Our first clue is what Darryl does. He explores fitness, just like the name of his site says. In other words, he's learning. It's a journey. He doesn't think he knows everything already. Our second clue is what he says. His blog sums it up:
"I constantly question and educate myself .... I realize that what I do may not be the best way, but it is the best way for me at the moment and it is better than what I have done in the past. I constantly challenge myself not only physically but also intellectually. Just because it sounds right doesn't mean it is, and just because it is not conventional does not make it wrong."
This is classic growth mindset stuff. Exploring and learning is the whole point. Our third 
clue is what he doesn't say. He doesn't say his way is best, and he doesn't claim to have thought it all up himself. He makes clear that much of his work and skill are rooted in other people's work and skill. Humble and deferential, but endeavoring to kick ass at the same time. That's Darryl in a nutshell. You can't get more growth mindset than this.
What's with you?
Which mindset do you have? Let's review. You have a growth mindset if you believe that intelligence and skill can be changed and grown throughout your life. You have a fixed mindset if you believe that intelligence and skill are fixed, and that the way you are now is pretty much the way you’re always going to be. Here are the main characteristics of the two mindsets. Read them and figure out which one you have. As you read, keep in mind that the growth mindset and the fixed mindset are very real. They're the findings of famed psychologist Carol Dweck, from 20+ years of research. Generally, according to Dweck, people have one or the other, possibly with some overlap.
Growth mindset people are open to feedback of all kinds, whether or not it’s favorable. They see it as essential for learning and growing. So they seek it out and use it to keep themselves and their work on an upward trajectory. They practice and tweak a lot. They’re O.K. with being wrong and screwing up, because they see it as an opportunity for learning. That's what makes them open to trying new things.
Fixed mindset people are not open to feedback because they they take it as judgement. They believe they can’t change much, so they're very protective of themselves in their static state. To them, it's all about the result, not the process. They think they already know what they need to know, so they're not in exploration mode. When things go wrong, it's because of other people or the circumstances. They're very cautious about trying things and stretching themselves, because if they don't get the outcome they want, in short order, they'll see it as a reflection of who they are (as opposed to something they did that can be worked on).
In sum, the growth mindset is hugely optimistic – it’s all about learning and growth. The fixed mindset is brutally pessimistic. It’s all about judging and being judged.
The takeaway
If you're already in a growth mindset, great. Stay there and build on it. If you're in a fixed mindset, don't worry, you can switch into a growth mindset just by "putting yourself in one" (paraphrasing Dweck). It's pretty easy. Just knowing about the two mindsets helps you spot it in yourself (and others). You'll start catching the fixed, which you can switch into growth. Over time, your perception will change - of yourself, others, and the world. Thats when life gets fun.
Onward
From my own exploring, I've taken all that I've found so far and distilled into 4 essential principles that drive change. Together, they comprise a model you can store in your head and use anytime you want to make a change in your life. The first is Mindset, which we've just gone over. In future guest posts, we'll explore the other 3 principles. (For now, here's what I'll divulge: like Mindset, they also begin with the letter M: Motion, Mastery, and Measurement).
Over to you: Which mindset are you, fixed or growth? Now that you know about the two mindsets, can you think of some people in your life who are one or the other? If you have a growth mindset, can you think of some ways it's helped you? If you have a fixed mindset, can you think of some ways it's limited you? Are you open to switching? Let's talk in the comments.
Susan Alexander blogs at gooddisruptivechange.com 
You can follow her on Twitter at @SusanRPM4.