There is, in most large motors, a starter. In cars that starter used to be a hand crank. Now there's a battery-powered electrical motor. Once the engine gets going, it starts to produce power and will run on its own with sufficient fuel. But getting started can be very difficult.
The engine is a good metaphor for productive behaviors. Sitting on the couch watching TV doesn't take much energy to start, but when you get up at the end, are you energized? Writing, and other types of 'work', take much more energy to get started. But, like an engine, they can generate some of their own energy once you get going. Sometimes work is draining, but often enough, we can get up from having worked feeling energized.
This seems true of a number of activities that one would think were more draining than not: athletics often leave one feeling exhilarated in addition to being tired. Many kinds of work have a similar effect.
Personally, with many of these activities, I don't engage because I fear putting in the energy necessary for the activity. And yet when I do engage--I force myself--I do find myself rewarded. For me this is most clearly played out in my running habit. Or almost-habit. Often I have no desire to run and I feel too tired; when I get myself going on such days I promise myself I'll keep the run short. And often on exactly those days, I find myself having a great run and going farther than I would have expected.
Writing can be difficult. There is no question that writing requires a lot of effort. But writing--especially writing about something that you care about--can also return its own sort of energy.
Too bad there's no battery-powered starter for a writer.