You sure? The fourth largest army (North Korea, active personnel) wouldn't last more than two days against the fifth largest? Not to mention that NK's is by far the largest in the world in terms of reservists.
Re-learn the lesson that was the "Russian Steamroller" in WW2. Even an extremely large army that is poorly equipped will fare badly in battle. To play that game, you have to be willing to toss your people into the fire like coal and simply outlast the offensives until some other factor kicks in. That didn't go so well in the first half of WW2 for Russia, and the Germans knew how it worked and very nearly completed their push. When you're taking losses above 10:1 it's difficult to physically get that many bullet-stoppers up to the front line fast enough to slow down the offensive, regardless of the size of your army. Logistics were the Germans' downfall - resupply and weather. Russia had to flood the line with bullet-stoppers until they caught a break. And in today's wars, and especially considering the proximity here, that won't be an issue.
Lack of any significant airforce would be another issue, but that really is irrelevent because NK simply hasn't got much in the way of substantial millitary targets to take out.
If Russia were to decide to "step over the border" into NK (like they're doing elsewhere currently...) I seriously doubt it would come to war. It would probably end up like more of a "ok you little snot-nosed kid, you've had your fun but we've grown tired of it. Time for us to straighten out this disaster." Kim would find himself on a plane to Moscow shortly thereafter. But fixing NK would be such an enormous headache / liability, would they really want to take on that burdonsome of a projecct?
Thing is, even with the best of intentions, NK is an incredible mess. Even if some well-meaning power had the ability to just step in and be handed the reigns to fix it, it'd be a tremendous undertaking. There's way too much wrong to try to tackle it all at once, it would have to be handled in stages. I can't imagine the borders opening up anywhere inside five years, regardless of who was at the helm. Right now the majority of people in NK have been brainwashed into believing their leader is a god, the entire world wants to eat their babies, and their military could conquer every army on earth. When most of your populace honestly believes this, you take on the very difficult job of saving someone that doesn't want to be saved. We saw that in Japan at the end of WW2, entire villagaes in the outlying islands committing suicide because of what the millitary told them the US troops would do when they got there. NK is exactly the same that way. We could drive a red cross truck loaded with food into a starving village over there and they'd come out of their huts with pitchforks, kill all the aid workers, and burn the truck, food and all, thinking it was poisoned. It takes quite a long time to fix that.