This reminds me of what one of the BBC websites says about writing. I remember coming across a BBC website which was all about teaching people how to write, with a focus on comedy. They are focusing more on TV writing of course but what they are saying and what you are saying applies to both books and TV (and also video games etc if they have a story). On that website it points out that many people know the start and end of the story and try to write linearly from point A (the start) to point B (the ending). However that doesn't work often because the middle is the interesting part, where stuff must start to get moving and maybe some curveballs are thrown at the main characters. They call the middle of the story the muddle because it is meant to get more complicated and be the "muddle" - the thing that doesn't let them go straight from a to b without dealing with c or do first basically. The muddle (middle) is what makes things interesting according to that site. This seems to be along the same lines of what you are saying when you point out that knowing the start and end but being lost in the middle is related to poor structure.