I could give them a pass for this. They might have been unable to obtain the ingredients for gunpowder, and over the six years that were skipped they could have used up what ammo they had. I believe, too, that there is a limit to how many times shell casings can be re-used.
I can't see walkers doing 30 miles a day, for that matter. While I'm at it, do walkers have to eat? I know they want to, but is it necessary for them to continue existing? If it is, migrating makes sense.