This question has garnered a lot of comments and a significant amount of effort to close it. If I had to guess from reading everything, it's because the effort needed to create this much detail on a march is significantly beyond what any group of people of any intelligence or tech level would ever do.
But, the real problem is why are they doing it?
Life tends to follow the path of least resistance unless forced to do something else. From that perspective, people are going to be strung out in a line willy-nilly on more-or-less a first-come/moves-fastest basis. Our own history of migrations (past and present) tend to show that migrating groups can get strung out along miles of distance. After all, people are just trudging along.
Why would this change?
Attack or a perceived threat will force everyone into a group with the defenders on the outside and everybody else inside.
Hunting/gathering and the need to feed the group will cause it to develop small groups in "orbit" (for lack of a better word) around the main group, which may be (and often would be) strung out in a line.
Distress, such as a wounded person, will cause a small group around the wounded person.
But, wouldn't predators attack the group?
Not necessarily. Earth history suggests that animals (including predators) tend to shy away from humans because they're not a natural part of the surrounding. This would be very true with a migrating group, where the scent and behavior would be completely unknown to the local wildlife. The exception is the one predator who's so hunger he'll attack pretty much anything — but planning for the worst-possible-case example has always proven economically and efficiently foolhardy.
Therefore, this would only change if your world has predators that would attack indescriminately and somewhat frequently. In other words, if there's a reason to push the group together.
And there's a reason not to walk as a big ol' group
Anyone who has hiked as a group through tall grassy areas knows the value of a trail blazer. The trail blazer is the person at the head of the line who's cutting the trail. It can be hard work and is frequently traded off to give people a chance to rest. The reason this is done is so that the rest of the group can walk with greater ease, conserving energy.
Walking in a big group like you describe would be hugely tiresome because the trail is the proverbial thousand times wider than it must be. All those trail blazers must occasionally trade off with rested people, and there aren't a lot to trade off with when so many are involved. People would quickly regroup back into the line as it would be easier and faster.
This behavior is generally even true with areas of low brush, where a single winding trail through (for example) sage brush is easier to navigate than a whole group trying to walk over (or around) so many bushes. Frankly, groups like you describe are only efficient on asphalt and maintained lawns.
And unless your group has a remarkably strong caste system, "defenders" are just the strong (usually males).
Finally, it's actually pretty expensive to dedicate strong backs to defense. Nations can afford that expense, but migrating groups probably can't. Most if not all of the strong men and women will have some kind of armament and would be expected to act as defenders should the situation arise. But, generally, when not taking their turn as a trail blazer, they're helping everybody else along.
You're moving a small community, not a nation or an army. At a guess, everyone would be interrelated and the community would already have formed strong help-each-other tendencies.
The grouping you describe is unrealistic unless there is an outside force acting on the group to keep it in that order. The complexity of the design is unrealistic in all cases. You'd have defenders on the outside, everyone else on the inside. Little or nothing more. A group so large that the complexity you suggest might make sense would be using scouting parties as a primary defense, negating again the need for the circular group.
The grouping they would naturally take without an outside infuence is a strung-out line, with the trailblazer at the front, a few defenders behind him, the bulk trudging along, and a few defenders in the rear. The exception to this would be hunting/gathering parties for food or scouting parties.
I can easily imagine that you're putting the cart before the horse. Rather than building your community's rules and behaviors and then letting the design of this group shake out naturally from the story, you're trying to create the group from an "outside" point of view and force it on the characters of your story. Life doesn't work that way, and your readers will realize that you're forcing the situation.