Build a terrain for a UE5 Flight Sim

I require a very large terrain, made correctly (world partitioning) in UE5 for a cruise missile simulation game I am making. It should be at least 100Km x 100Km - it does not need to be overly detailed but it should be made correctly using a mesh/height map etc. It needs decent lighting and distance fog so the "edges " cannot be scene or a skybox that mirrors the style of the terrain. Ideally a mountainous region. There will bee extra work after this such as enemy and ally base design etc but that will second to the "base" world.