^Indeed. Slow and steady wins the zombie race. I mean come on, if a person is travelling exclusively on foot, I can't see that those walkers are going to slow her down *that* much. It's not as if she's liable to be able to sprint all day if only she didn't pull those two along.
Plus of course, being at a walking pace is part of the disguise. You can be pretty sure that if she just suddenly started jogging that every walker with a line of sight would instinctively chase after her...then the ones that seem them dash off instinctively follow and so on and so forth and before you know it, there'd be a bloody horde slowly running her down.