I think it's best to incorporate your own natural rhythms, to the extent that you can and still complete your work. You said you can do 6 hours of concentrated work, so after that, why not take a break for as long as it takes to refresh and then go back and do anothe hour or two work, on the less demanding, "administrative" tasks.

By the end of the week, you may find that you don't "need" to work additional hours on the weekend. . .at least not every weekend.

Also, experiment with the times of day that you are most productive. You may find that at different times of the day. And remember that you may not always need 40 hours in a week to feel productive. Other weeks, you may have the energy, desire and drive to work for 60 hours.