I'm trying to make a map with flyers in it, but the annoying thing is that the texture squares of the terrian are really noticable at a height. How can I solve this problem by having the terrian look realistic, if it's even possible.

If the map is mainly going to be seen in the air, lower the texture tilerange. Lower numbers mean less times the texture is repeated across the terrain, and vice versa.
Pro-tip: The larger the terrain texture resolution is, the better it will look. I don't accept any terrain texture that's smaller than 1024* nowadays. I always end up lowering the tile range to match the size of the resolution. In the image I showed you, my 512* terrain texture had a tile range of 32. When I got a new texture that was 1024* I turned it down to 16. This won't always work, sometimes you need to tweak it till it looks right.

Anyway, let us know if you need help with anything else.