I think l9k might have been making a more subtle point: maybe motivation/willpower isn't quite so different from the other required personal attributes (in this case, physical fitness) as we tend to think. Perhaps because it can't simply be switched on and off like a tap, but requires training analogous to physical exercise; or perhaps for deeper reasons. (Going down this road pretty quickly leads us into a thicket of ambiguities and philosophical puzzles around 'free will', so it might not be very productive, but it's still worth bearing in mind occasionally.)

Thing is, when looked at in the context of a whole lifetime, motivations change, and often radically. I have friends who couldn't have imagined themselves running a half marathon or a marathon who eventually went on to do so. 10 years ago I couldn't have imagined myself doing an obstacle race, but then I started working out[1] and, hey presto, the impossible became possible. I still don't want to run a marathon, even though I know I could, because of the compromises it would force on me in terms of strength training, depending on what kind of time I'd like to finish in.

[1] I couldn't have imagined working out either but at 36 years old I found myself getting tired out walking upstairs, and having a torrid time with any kind of reasonably intense physical activity, and realised I needed to do something about it to avoid serious health problems later in life - and possibly not that much later.

I think you are confused. "Not being arsed" to do something is the same whether it is running a marathon or a writing a book.

He cant be arsed.