Is it the act of just moving? I'm assuming this also goes for cycling? I cycle to work (5 miles each way) and the 25 minutes each way have sections where I can mentally focus on other things and just let my body ride.
I can pretty much set myself up for the work that day in the morning thinking through issues very deeply. In the evening, I find it clears my head and lets me think around any issues I've had during the day or just get myself prepped for any side projects I'm working on that evening.
I like the idea of a walking meeting however the dynamics of having 5 people walk together and be able to have good conversations might not work. I'm guessing 2-3 people maximum would work.
How does this relate to standing desks? Standing desks with treadmills underneath them? ;)