What I meant to say: IMHO, the sense of 5.15ff demands that PERIPATEITE has to
be supplied again as an imperative. "Watch out that you walk (PERIPATEITE as
an indicative) carefully, walk (supplying PERIPATEITE as an imperative) not as
fools but as wise, . . . ."
There may be a few other places in the passage where you could supply the
imperative PERIPATEITE (and remove it from the place where I've supplied it),
but the indicative PERIPATEITE becomes an imperative PERIPATEITE before 5.21.
And the participle in 5.21 is just carrying out the supplied imperative.
Grace and peace,
Perry L. Stepp, Baylor University