That "how'd that happen?" feeling

I was rewriting a section of a chapter today, working through subsections in kind of a random order. Getting to the end of one subsection, I turned to my to-do list, crossed it off, and searched in vain for the next bit I needed to rewrite. There wasn't one. The whole section was finished. Kind of a weird let-down feeling, if you don't know it is coming.

This was only one section of one chapter, but it got me wondering whether that's the way this whole dissertation thing is going to end. I'll finish up rewriting one insignificant subsubsection, turn to my list, and realise there is nothing left to do.

More likely, though, judging from the experiences of other grad students in our department who have submitted in the last few years, "completing" the dissertation will be a matter of frantically doing as much as I can in a never-ending series of improvements until my time-limit runs out and I have to hand the thing in whether or not I am happy with it. Or is there actually some way to come up with an achievable concept of what "finished" would be, work towards it methodically, get it done, and hand a dissertation in that satisfies some definition of completeness?

No, I didn't think so either.