There's so much interesting stuff that happened at other times in the Dark Tower story that we don't hear about.
We learn a little of his childhood, and when he was out at that place with Susan. He returns from that, and the next scene is Jericho Hill, like a few years later. Then, after that, the next scene is him chasing the man in black across the desert.
He could probably write a full novel about the fall of gilead leading up to jericho hill (it's very important, it sets him up for his quest, and we only get a 3 page summary of what happened). We know the gist of it, but how did it get to that? How did Roland survive?
And there's probably a dozen novels' worth of material between roland beginning his quest, and him reaching Tull.