A definition and word history from Dictionary.com.

lacking in vitality, imagination, distinction, etc.; commonplace; prosaic or dull: a pedestrian commencement speech.

From the word history, we see this sense of “pedestrian” predates the common modern usage of a walker!

1716, “prosaic, dull” (of writing), from L. pedester (gen. pedestris) “plain, prosaic” (sense contrasted with equester “on horseback”), from pedes “one who goes on foot,” from pes (gen. pedis) “foot” (see foot).

Meaning “going on foot” is first attested 1791 in Eng. (it was also a sense of L. pedester). The noun meaning “walker” is 1793, from the adj.