It'll likely be a long, drawn out campaign, akin to when Russia went into Afghanistan. The Ukranians are there and willing to fight, and would engage in guerrilla tactics, which leads to a high body count. Russia is a conscription army, and when things turn ugly and long-term, a conscripted army is absolutely what you don't want to have. Body bags coming back to Russia and angry Russian mums, basically.
It's what happened in Afghanistan, and could well happen here if he goes in. Not saying it would, but it could. And that would spell the end for Putin. He's not daft, either, and though he has a planetary sized ego, he'll realise this.