Rectangular Terrains

I am trying to make a level with a terrain that stretches out a great distance in one diretion eg. 1000 metres wide, but 10000 metres long.

However, because the terrain uses a square heightmap, when I start to sculpt the terrain, it is all stretched, and this is not practical; even if I were to make a sculpting brush that is “squished” to compensate, it still isnt practical because it would give me a super-high density across the width, which is wasteful of resources.

Is there a way to make rectangular terrains?
Also, I dont want to splice several terrains together, unless there is a way to do it such that all terrains can be sculpted as one without seams.

Try using 10 terrains.