The Middle of What?

[alatar]

So are we saying that 'Middle Earth' is middle because it is betwixt fire and ice, sea and shining sea? When was it established that there was a sea to the far east? Or did everyone just take Aulë's word for it? Was it one large continent like Eurasia?

I would conclude then that, like some of our own ancient stories, Arda too once was a flat table-like surface which then later became a globe. Did Tolkien create it this way to resonate further with our histories?