Spoiler on Michonne in the new Playboy. It explains who the two zombies are and why they're on chains.
If the zombie herd was heading out of the city, why wouldn't our group want to be BEHIND them? Staying in front of them means the herd is always chasing them; being behind the herd should give some rest.
And shouldn't the herd be decomposing more and more? I can understand that there could be new zombies as people die, or evidently, the zombie-flu takes them over (a la Shane).