I know that the geography of mid world would not necessarily correspond to our own geography. While I was reading the first 4 books in the series, however, I couldn’t help but try to find some correlation. That being said, here is my theory on the path that Roland travelled. The Mojaine Desert is the equivalent of the Mojave Desert. The mountains Roland and Jake crossed were the Sierra Nevada mountains. The Western Sea is the Pacific Ocean, and the events of The Drawing of the Three take place on the California coast heading north toward Oregon. The Wastelands I think takes place starting in Oregon and heading East across Idaho, Montanna, South Dakota, and down into Lud, which is New York, but located where St. Louis would actually be. From their they take Blaine to Topeka. After that, it gets difficult to track the ka-tet’s movement.
As far as Wizard and Glass, Hambry in Mejis seems to be part of Mexico. Their is a Clean Sea, which could either be the Gulf of Mexico or the Sea of Cortez. Since Roland doesn’t mention being near Mejis during the Drawing of the Three, I think Hambry is in northeastern Mexico, say around where Matamoros, Mexico is. Since Mejis is in one direction, and a desert/waste is in another direction from Gilead, I think Gilead is in Texas, possibly San Antonio or Dallas, where going south would lead to Mexico and going west would lead to the desert in West Texas.
This post is longer than I thought it would be. Please respond with your ideas on where I have gone wrong, or if trying to correlate the United States with Mid World is just completely off base.