My recommendation then would be to "chunk" the terrain; take your current terrain and divide it up into several parts. I suspect the reason your texture looks low res is that the model is too big for the texture.

I'm not much use on the sidewalk bit though... I suppose you could edit the height-map on your terrain to make a sidewalk above the terrain, but it wouldn't be an object in that case.

(Edit) I should probably explain chunking better:
Say your terrain is 1024x1024 and the texture is 512x512. Your texture is being stretched to double its size. If you divided the terrain into 4 chunks, each chunk would be 512x512. You would seperate the terrain into 4 parts too, but each part would continue to be 512x512, so now the image isn't being stretched.