I have the same problem, I collected some answers:
If the textures appear to be the correct sizes, but are just super blurry. Look at BaseMapDistance (under the far-right Gear menu in Terrain.) If it somehow got set really low (or your terrain is huge) Increasing that may create a “crisp” area.
“Modern” terrain engines pre-make a super-low-res image of the entire terrain. For anything that counts as far away,
they just use that low-res baseMap (for speed.)
Hey guys, same thing happed to me a moment ago and I think I figured it out.
If you have DirectX11 turned ON, you HAVE TO have a “Nature/Terrain” material assigned to the terrain. If you don’t,
the textures go blurry.
If you don’t want to have a material assigned to terrain (because for example you use a TriPlanar script), turn OFF
DirectX11 in Player settings and reset Unity. Textures go back to normal 
Hope this solves your problem 
I had this same problem but then it turned out to be the shaders that I was using were bad. I selected the terrain in
the hierarchy then I went to Assets->Select Dependencies. Then I deleted all of the shaders and it worked perfectly. I
hope this helps.
Have a look at Project Settings > Quality and the import settings of your texture.
inside the terrain go to settings and change the base map distance to 0. Inside the quality settings set the quality
to max, go to your texture and change resolution, set to BiLinear, if none worked delete the TerrainAssets folder(or
whatever texture pack ur using) and reimport it
Try them!