You think about the plot and sub plots. You think about the characters and their motivations. You think about the weather and the time of year the novels take place. And you think about the locations. In fact you think about everything there is to think about when writing a novel, and rather than spending my time sitting in a room and staring out of a window thinking, I prefer to do most of mine while walking. Not only does this help to keep me fit but it's a well known fact that exercise also improves one's mental powers.
In addition, thinking while walking has the added advantage of getting good location description, as well as providing descriptions of the weather. It also means coming across some very interesting characters, who might inspire a character/s in my novel. These aren't necessarily main characters but usually secondary or walk-on characters. It might be just a glimpse of someone walking over the Downs, strolling along the promenade between Sandown and Shanklin on the Isle of Wight, or striding out along the Hayling Island Coastal Path. Or it might be someone I end up talking to on the train, or in a cafe or shop.
The other day, on the train from Shanklin to Ryde (after walking seven miles), I made a study of people's toes and found it quite fascinating. Of course this can only be carried out in the summer months in England. There were deformed toes, tattooed toes, dirty toes, painted toe nails and toes with long nails, (mine were safely ensconced in walking boots). As a creative writing exercise I could simply list these and ask students to form visual descriptions of the owners of the toes and then pen character sketches of them. It would be fascinating to see what the students came up with.
Then I made a study of the faces belonging to the toes, surreptitiously I might add, I didn't want to be accused of staring. There were youthful faces, worried faces, alcoholic faces, and vacant faces. And that's just the edited version of the descriptions I tucked away in my mind. It's all good stuff for the next and future Andy Horton marine mystery crime novels.