The 'Mid-landers' are a cultural group that stretches from south-eastern Pennsylvania through to Iowa and beyond to Colorado.
Actually, Woodward's Midlands looks like a gerrymandered monstrosity that doesn't appear to cover the majority of any US state except Iowa and maybe Kansas and loops north through the Dakotas to include parts of Manitoba and Ontario.
Looks like Woodward is retreading Garreau's Nine Nations of North America.