There are a few reasons why I think there are good reasons for drawing some parallels between Middle Earth and the real world, here, again with the proviso in mind that we're not just making one an allegory for the other:
- We are told in The Silmarillion that beyond the Sea of Rhun is the Sea of Helcar, another big inland sea. Helcar would seem to be rather like the Caspian to Rhun's Black Sea. We're also told that beyond that there is a range of mountains called the Red Mountains, which seem to me like they ought to be the Caucasus/Urals.
- Rhun is inhabited by Easterlings, who are apparently nomadic or at least semi-nomadic. This implies both a steppe landscape and also cultures something like the Cossacks, Alans, Huns, Magyars, Bulgars, Scythians and other tribes who circulated on the Pontic Steppe at various stages of its history.
- The Numenoreans apparently also had colonies around the Sea of Rhun, which suggests to me something along the lines of the Greek colonies on the Crimea, which culminated in the 'Greco-Scythian' Bosporan Kingdom. You can imagine Numenoreans still living in these colonies in the Third Age, perhaps with 'Numenoreanised' Easterling populations as well.
- Humans purportedly came from this area originally and this seems to chime, in my mind, with the Pontic Steppe being the original home of proto-Indo-European speakers in the real world.
- Well, it looks flat, doesn't it?