Ouch. That changes everything indeed... How did you manage to beat down a player Russia in the first place?? Is it a skilled player?
In your situation perhaps you could try focusing on Iran (trying to play defensively, losing as little units as possible) and continue to defend against Russia without a peace treaty. Defend ultra carefully against Russia, create huge garrisons everywhere and leave only a few units in the cities so they would have to fight for the land (basicaly you would just keep him occupied with this while you fight Iran.
Then while playing defensively against Iran, seek as many alliances as you can around yourself and Iran and try to fight them until they either accept a peace treaty or until you annex them (hopefully before Russia is in your capital). The chance of success is slim, but why not try?
Even if you lose, it will be fair to the Russian player that you played to the end.