What was the timeframe of the rumbling again?https://i.imgur.com/lIq8FNO.jpg
from this we know the radius of the walls, 250, 380 and 480 km respectively (IMO it doesn't make sense with the timescale shown pre timeskips but that's the lore what can I say), that gives almost 7,000 km assuming they're all in a line shoulder to shoulder... It can literallly start at the whole eastern coast of Marley and just walk east->west in a week, then it can target other continents
Also we don't know about population distribution, IRL early 20th century Europe had disproportionally high population due to having started the demographic transition earlier, if Marley is the historical center of the Eldian empire and the world superpower it may have disproportionally high population, particularly if it's over a whole entire continent. We also know their army is 1,000,000 strong without mandatory conscription, that's much more than the US or UK, countries which didn't have mandatory conscription, had before ww1, implyign a much larger popultion.