You make me smile, nearly 10 years as solo consultant too. For the past 5 years my "week of work" is 25 billed hours and for extremely hard problems (reworking old complex Fortran thermodynamics code) this goes down to 15h/week. Above that, I cannot sustain the rhythm more than two weeks.
The non-client work is done on an "energy and motivation" basis, so I accept that some weeks are good but I can do nothing two weeks in a row.
Ha, "energy and motivation" basis sounds about right for non-client work. I'm finding that getting smarter about what work I can outsource (e.g. bookkeeping to bookkeepers, tax preparation to CPA/accountant) is helping me out tremendously. It's very tempting to keep doing things because "I know I can do that", but forcing myself to let go of non-core/non-strategic areas is hugely empowering and helps with the balance issues and optimizes energy / context switching.
The non-client work is done on an "energy and motivation" basis, so I accept that some weeks are good but I can do nothing two weeks in a row.