Do we know if these zombies need to digest food? Bicycle girl may not have had much left of her digestive tract, but she was still going for it. Obviously the undead need to get energy from SOMEWHERE, but is it from the act of eating the living? Has the comic book covered any of this?
One wonders, if this is a virus, that the virus iteself metabolizes eaten flesh once it gets to the esophagus. Then the virus would collectively be able to animate its host body after doig so. It's not like zombieism is an exact science, but as far as we know, it's not black magic in The Walking Dead.
As for speed, I was thinking that lots of the dead lurkers would simply able around the area they reanimated, such as the FEMA facility, until some source of food came along. And if it's been a couple months since anything living was in the area, they may have been getting desperate for a new food source, but without the wandering instinct that the herd zombies have, they were ambling in wait for some fresh meat.