I have not seen people who could work every day for 8 hours in a row without losing motivation. Any monotonous work is exhausting and reduces the concentration and efficiency.
I have tried many methods, including popular GTD-like ones. I will not say that they "do not fit" - each of these methods has its own benefits and drawbacks, so my personal method is a kind of compilation of all known and tested by me earlier.
The basis of this method (I call it "practical planning") lies in an understanding of the effect of 10,000 hours: it's enough to spend about 10,000 hours on any activity to become a world-class expert. Translated into work days, it takes about 5.5 years of 5 hours a day, seven days a week - a quite sane mode for your favorite tasks.
The final goal is not so important (if you spend enough time, you will become an expert anyway). What's more important is to force yourself to do this work on a regular basis, in marathon mode. There must be no goal to work for 12 hours a day (I tried it and I must tell you that it quickly burns me out). The main goal is to organize your working day so that the execution of this plan will be easy and natural.
Motivation and will power are at their peaks at the beginning of the working day, so the mornings should be dedicated to practice, not to the planning or note-taking. In theory, it seems that it's a good idea to plan your morning in the evenings before. In practice, it turns out that long to do lists demotivate, so evening planning is not just useless, but seriously harmful.
I organize the mornings of each working day using planning/practice mix:
First thing in the morning I ask myself if I have some good ideas and motivation to begin work right now? If the answer is "yes", I start my work immediately. There is nothing more valuable than good ideas and motivation to act on them right now.
If i feel lost, don't understand what to do or just have no motivation to work, I open my diary and begin free writing, trying to refresh my goals and motivation. When I clearly understand my next step, I plan its execution, dividing the problem into the short actionable steps. If I see a step less than 5 minutes long, I do it right now, rather than adding it to the todo list.
Instead of one continuous process of planning and subsequent execution my mornings are broken up into short planning/execution stages, where each todo list consists of a maximum of 5-7 short steps done in up to 30-60 minutes in total.
At the same time, I don't try to work for exactly 5 hours, although I strive exactly for that. Instead, I appreciate the problem according to the principle: "If it's worth a full 5-hour day?" If I have done some task or a set of tasks on the scale sufficient for a 5-hour work day, I can safely put myself "check" and leave.
A small caveat: even if your motivation is high, it's better not to work for more than 5 active hours per day, especially for several days in a row. You will burn out really quickly, in just a few days, then procrastinate for a couple of weeks. It is better to work gradually, but regularly, on a daily basis.