I think YMMV is not only hte key statement, it's true for all of us.

Good prior observation in this thread is that the speed of a group will be the speed of the slowest member. Since it is only DW and myself, and we both climbed up Mount Washington in September (4 miles, 4000 feet up and carrying a pack), I have no question we could make that kind of a distance over flat ground. Our packs would likely be heavier, but the terrain would not be as difficult or with as much vertical. The first day, I think 10 miles is doable for us. If some of my family members were involved, well, Mom is pretty much not walking out of anywhere.

The second day is a different story, for the walkers. If you have a group, many will be sore and tired from activity they are not used to. Someone is likely to have blisters, someone may be nursing an injury such as a twisted ankle. If you get everyone going, day 2 should probably be dialed back from Day 1. If no one is hurt, someone is more likely to get hurt walking on those tired legs.

Thereafter, speed should pick up, all else being equal. But from what I've experienced and seen, making 15 miles a day is strenuous throughout most of the northeast. If you are not in good shape at the start, it's going to take a while to get up to making that kind of speed.

In the end, I don't think there are many who are going to get up to walking more than 10 miles a day, unless they stick to it for weeks.