Sheepishly I'm a huge fan... of walking.

I can be pretty stubborn.

Something that I've been stubbornly wrong about it that walking is legitimate "exercise". I've always thought it wasn't strenuous enough or cardio enough to make a real fitness difference. Even with all the recommendations from "authorities" to walk more, I had my own theories about it. I thought that encouraging walking was just a "baby-step" to help unfit people get closer to being active. Maybe the thinking was if you could get people to walk more, they might really exercise someday.

Well, through a circuitous route I've landed myself in the "walking is better than" running, or cycling, or jogging, or _____________ camp.  When I take a brisk walk, I find that my insulin kicks in and works better, faster. If it can be seen on a CGMS I'm guessing the same things are happening in everyone that walks. For me, seeing the numbers at work has been very eye-opening.

What does walking have that other forms of activity might not?

It's low-impact. You aren't likely to develop injuries from repetitive actions that your body isn't ready for. Since you walk your whole life, it's the most natural way for you to get moving. (One summer I injured my achilles tendon from doing lots of dog-walking in flip-flops, but that was unusually foolhardy.)

It's accessible. You can walk in any clothing you have on (caution with flip-flops recommended). You can walk wherever you are. You can walk with people who don't normally "exercise". The barriers to getting it done are very few.

It's easy, and the heart rate is spot on. How many times have you done a serious run or bike ride and found yourself exercising at a level that makes you feel lousy and sore, and not look forward to doing it again? For me, if I exercise at a heart rate of over 155 bpm, I'm officially working too hard. I find it very hard to dial myself back to that level when I run or cycle. When I walk at a fast pace I don't think it's possible to breech the threshold into unhealthy exercise. We never return from a walk and think "Wow, I'm super exhausted!" From talking to others, I have found that most people tend to exercise above their target heart rate zone, and often are reluctant to get out there for another workout, because, frankly, it was uncomfortable. For this reason many athletes use heart rate monitors to keep themselves in check and keep from working at a rate that is too high. For me walking is always in the low end of the target heart rate window, and I don't need a heart rate monitor to tell me how fast to walk.

It reduces stress instead of causing it.
Excessive or overly strenuous exercise can cause you negative stress that can oftentimes be worse for your body that better. Cortisol (stress hormone) release from strenuous exercise is conter-productive to most people's fitness goals. Walking has very little risk of working against you this way.

Conditioning. 
Contrary to what I had previously thought, walking regularly this season has improved my readiness for other activities. Last summer when I began stand up paddling I found that I could only go out for 15-20 minutes before my legs got too shaky to keep going. This year, on my first day out I was able to go out three different times for about an hour without feeling fatigue in my legs. I think it has a lot to do with the regular activity throughout the winter that prepared my legs for long periods of balance work. That was unexpected. 

I'm not saying that everyone should stop doing the exercise that they find stimulating and fun, but that for baseline activity walking is safe, easy, accessible and useful. I began exercising much more frequently since I moved walking onto my exercise menu. If I could cycle, rock climb, cross-country ski, stand up paddle, hike, and kayak everyday I would be ecstatic. Until I retire and live in such a paradise, going for one or two 20 minute walks a day has been incredibly helpful in increasing my insulin sensitivity and correcting BG that is headed out of range.