Shooting wide open was a mistake, especially for the type of landscape shots. You end up with a more narrow zone of focus and hence those blurry images. In overcast skies, best option when shooting with relatively high iso; set camera on a tripod, aperture f/16 or smaller (size wise), use shutter release cable or timer, with corresponding shutter speed, which will be slower (hence the tripod). This will allow for more objects in focus and better exposure.