Thats what the geohashes are for. Once each channel gets a bunch of people into it, you can go down to more local channels until you get to the point where the mesh is actually usable.
So you might start out with the region channel covering 1,000 km, and when that gets like 1,000 people in it, you'd switch to a more local channel covering like 40 km, and when that gets full, you'd switch to a more local channel covering like 10 km, etc.
