What's the easiest way to fix this streched texture?

Based on the sudden slanted direction of the pattern, I would assume that the object has a UV map that you are unaware of (it may be implicitly generated). If not, please describe how you attached the texture to the model, in Unity or another app? I would definitely double check the model's UV. The grassy triangle immediately to the bottom left of the building in image one tells me it is not a uniform mesh (straight tidy tiles), so I doubt Unity has a smart way of attaching it. Try UV mapping it by looking from above and hitting project from view.