This won't work at a company that inherently values long hours, yes.
And it's much easier at a job you've been at for a while, yes, because then you're much more valuable in a whole bunch of ways. It is doable at new jobs, but that is much harder the first time around.
So you have the right plan:
1. Get a job at company that isn't obsessed with long hours.
2. Stick around for a year so you're extra-valuable.
3. Negotiate shorter workweek.
You will then have easier time negotiating at later jobs, especially if you don't quit ("thanks for the offer, I'd love to come work for you. however, at current job I work 4 days a week... do you think you could match that?")