Generally you want to think about focus in terms of what you want to appear sharp. If you want multiple elements of an image to appear sharp, you'll focus about a third of the way into the scene. You'll also want to consider both aperture and focal length. For a lot of the image to be in focus, you'll want to select a higher aperture - like f/11 or even f/13. Focal length matters because with longer focal lengths, depth of field is shallower so you have less of the image in focus. With a wide lens or low focal length, most of the image will be in focus even at f/5.6 if you focus on the right place.
In this specific image, you were at 55mm with a DX camera - an equivalent of 82mm. It looks like the bushes are about 50 feet or so from the camera. Assuming you wanted both the bushes and the bridge to be in focus, you'll need to focus on the bushes (there really are not many choices, but you could focus just past the bushes in a perfect world - maybe 65 feet instead of 50 feet - and the bridge would be more in focus. If you focus on the bushes, you'll need an aperture of f/9 at 55mm. If you reframed the image to a 35mm focal length, you could focus closer to the camera and have the entire scene in apparent focus - 36 feet at f/5.6 would work. And if you framed the image with to an 18mm focal length focusing on a foreground element 10 feet away, the entire scene would appear in focus at f/5.6.
As a practical matter, for landscapes I use f/11 as a starting point and adjust as needed. I also try to focus on something relatively close in the scene - especially with wider lenses. You are far more likely to naturally focus on something too far away than too close with a wide lens.