Russia has absolutely no hope of sustaining a prolonged war as with each passing year they grow weaker. Their economy is in shambles (seriously, a drop from $2.2 trillion in 2013 to around $1.5 trillion today GDP), their diplomatic position is weaker than its ever been, and their reputation across the globe is in the gutter. The one thing they had going for them were their relations with China, but those have been chilling for ages now and there's absolutely zero chance China would actually engage the US in a war on Russia's behalf, both because their military is no where near prepared for such things and because the US is essential to their continued growth.
Right now their best hope would be to deliver a kind of non-nuclear knock out punch early (unlikely) or keep the war localized in the form of a "proxy war" (a bit more likely), as they would have zero hope against a US/NATO coalition in the long term. They have an exceedingly strong military and navy, but those matter little when after a few months you can't afford to keep them in the field, and much of your territory across an enormous expanse is kept in check via expensive occupations (Crimea, Chechnya etc). Russia's erratic, reckless behavior as of late are signs of desperation, not strength.
What would almost certainly happen is Russia would be chased out of its positions abroad and peace talks would begin once they were largely contained to Russia proper, as at that point there'd be no benefit to anyone if the war went any further. People certainly aren't interested in invading Russia, fighting those large armies, or pushing them to a desperation point that they might actually contemplate using nuclear weapons (wouldn't put it past Putin), and Russia's economy is already on the brink without a full scale war as it is.
There's only two real reasons Russia has been so adamant in their defense of Assad: Assad leases them their only year-round warm water naval port in the Mediterranean and is the only nation in an important region of the world that they share close ties with (the warm water naval port is also the primary reason Putin wanted Crimea, as it has one that is available for use for most of the year in the Black Sea). If war broke out and the US/NATO forces immediately destroyed Tartus (the leased naval port), and you can be assured they would do that virtually day 1, and thereafter they actively fired upon the Assad regime, most of the motive for the war would vanish in an instant. For that reason I can't really see it ever venturing much beyond a short proxy war, though no doubt there'd be all kinds of threats and bluffs thrown both ways.
The only country that would actively benefit from a full scale war, after all, would be a neutral China sitting back and watching. All countries involved know total war is not in their best interest, which is why we've been fighting proxy wars for decades now.
The western world is built on the dollar economy which is why they normally use sanctions and such against countries to hit their economy the problem here is Russia not only have their own resources they're also well develop and are very technically advanced which is why when they were hit by sanctions before they still carried on like normal as they don't need really need to import anything which makes a war with them disastrous and many western countries are not in the same position, not only that many countries are starting to lean towards moving away from dollar trade, economy is one thing but actual practical situation is another. When we hear reports of their position blah blah it becomes the same ol as we've been hearing that for decades now.
The allied forces are the ones not prepared to deal with Russia because anytime an opponent who has been able to strike at them directly pops up we've been taken to the cleaners near enough and guess who ends up tearing up the opponent to shreds? Russia, a load of groups have gone there to fight them and a load of them have been put in the ground, fact is they have an huge effective Arsenal and a location that is hard to engage, if Russia were as weak as some of you are claiming believe me our governments would have taken direct action when Ukraine was invaded but they know its a hard fight so have to just bark hard to make it look like we can deal with them.