The walk itself wasn't that bad, taking about forty minutes to cover the four kilometers of the Appian Way between CM and the catacombs. But the Via Appia certainly wasn't built for pedestrians. Or maybe it was built for nothing but pedestrians and it is the cars that are the odd ones - either way, it is not a place for something soft and splattable in the presence of things hard and motor driven.
By hugging the wall to the left or right depending on which side could offer most cover (or sometimes, even a half-meter sidewalk), I finally got down to the catacombs.
(Cabbed it back.)