I understand what you mean about writing taking time, and enthusiasm. If it feels like a chore it won't turn out well, I know.
When I write a book, (I'm up to my 4th), I plan the whole thing and write it down in a passion. Some scenes were the originals in my mind, so I write those out in full but might skip over the connecting bits in a few sentences (filling those out later, of course). Then I separate out what I've written in the plan into 25 chapters. Then I know what's happening and when. Then I can relax and get started! :-).
The details will come as you're writing it. Some chapters might seem short after your first draft but don't worry, the second draft will sort that out. Some chapters will be too long, so they can be split into 2.
My first draft mainly focuses on what happens to who and when. 2nd draft is what the characters feel, see and heard (and smelt) and the 3rd draft adds the spice. Eg. The clever sentences and observations.
Writing a book is daunting if you look at the whole mountain, so just treat it as a series of drafts, and before you know it, you'll be the author of a book. :-D