I didn't start reading the book until the series was done. I ended up reading the version of The Gunslinger that was an extended version. I remember that Roland catches Marten or whatever his name is with his mother and then out of rage challenges Cort to his test for his guns. Like you said Wizard and Glass is mostly a prequel that goes into more depth why Roland is doing what he's doing. But in the first book Stephen King makes it sound like his original purpose is to chase after the man in black and kill him for what he did