Not that tough any more. Alexander Solzhenitsyn was saying thirty years ago that Chinese expansionism into Siberia was the really serious long-term threat to Russia, and he is right. The Chinese have always contended that Siberia rightfully should belong to them. There is plenty of oil and minerals there, plenty of timber, plenty of water, plenty of land (cold though it may be) -- and it is contiguous to their existing territory.
At the rate present trends are going, I don't see how Russia can continue to hold Siberia against the Chinese for more than another 2 or 3 decades at the most. Given China's 5000 history, that is only a blink of an eye.