Screenplay structure is so tight and formal it's almost like poetry, an analogy I found very useful.
My novel (http://www.amazon.com/Darwins-Theorem-TJ-Radcliffe-ebook/dp/...) was developed much more organically.
I'm an experimental physicist, and the process for me was more like setting up an experiment, from early ideas to failed prototypes to little side explorations to a final result with (hopefully) all the loose ends tied neatly away where the reader can't see what went into the making.
I found Stephen King's book on writing to be one of the best for understanding the organic process of creating stories. If you haven't read it, I'd strongly recommend it: http://www.amazon.com/Writing-Stephen-King-ebook/dp/B000FC0S...
I agree with what you're saying... a novel is the end product, a screenplay is a blueprint. It took me 4 years to write the novel (admittedly very on and off) and ~3 months to write each of the screenplays. Hopefully my next novel will be faster.
What was your experience of writing both formats in terms of time and effort?
My novel took 5+ years of blood, sweat and re-writes.
Writing a passable screenplay took five days, about six hours a day (evenings 6 - midnight). I don't expect I'll ever sell it, but I'm as certain as anything that it's better than a lot of screenplays that have been sold.
I'm mostly a poet, and love structure. Once I understood the structure of a screenplay, it just happened. Novels in contrast are these huge unstructured messy awkward wobbling teenage flailing morasses. The lack of structure, the lack of constraint, the lack of form is something I struggled with for a long time, and while I expect my next effort won't be quite as painful, I don't ever expect it to be easy.
"Form is liberating", as the engineer's proverb goes. Novels don't have it, and are therefore much more work.