I used to very strongly feel this way, but recently I'm definitely been choosing to put in that much time (and I mean actual productive time), despite being at a company/team that puts a strong emphasis on work-life balance (there are a lot of parents on my team and they're out the door at ~4:45 to pick up kids, etc). I think it might have something to do with the fact that I happen to be particularly motivated and excited about what I'm working on right now (more so than basically ever before in my short working life). As you said, it may not be sustainable, but I've been doing it for about 8 months now.
It makes me wonder if the real issue is being able to do 40 hours of (relatively) unstimulating work, vs just 40 hours of work. I know for sure that I can't handle a tedious, brain-dead task for even half an hour without needing a break, and when I've had roles that were somewhat challenging but less interesting in, I killed a decent chunk of my 40 hours reading articles, etc.
It makes me wonder if the real issue is being able to do 40 hours of (relatively) unstimulating work, vs just 40 hours of work. I know for sure that I can't handle a tedious, brain-dead task for even half an hour without needing a break, and when I've had roles that were somewhat challenging but less interesting in, I killed a decent chunk of my 40 hours reading articles, etc.