It's the same for me, I work in bursts. Sometimes I will have 30m-1h of not very productive time (reading blog post on web, going for a walk etc) but then I will have a burst of productivity when I hunker down for couple of hours and do a lot of work.
Obviously it isn't constant 8 hours of heads down, in the zone coding, there are things in between and also meetings and planning often takes time. I used to work longer before but as you say, that is not sustainable for a long run, you can do it for couple of years before you burn out.
I also used to do quite a lot of extra work in the evenings and weekends when I had sudden inspiration / urge to refactor or improve some piece of code. But I don't do that anymore and instead use that free time for myself and my hobbies :)
If I feel an urge to write some code on Sunday, I will work on one of my open source projects or contribute to some other projects out there if they interest me or I will experiment with new tech to keep my skills up to date.
Obviously it isn't constant 8 hours of heads down, in the zone coding, there are things in between and also meetings and planning often takes time. I used to work longer before but as you say, that is not sustainable for a long run, you can do it for couple of years before you burn out.
I also used to do quite a lot of extra work in the evenings and weekends when I had sudden inspiration / urge to refactor or improve some piece of code. But I don't do that anymore and instead use that free time for myself and my hobbies :)
If I feel an urge to write some code on Sunday, I will work on one of my open source projects or contribute to some other projects out there if they interest me or I will experiment with new tech to keep my skills up to date.