This is very graphic. The Greek in the first part of this verse runs thus, according to the best authorities: καὶ εἶδον αὐτοὸς ὑπάγοντας καὶ ἐπέγνωσαν αὐτὸν πολλοί: And they—i.e. the people—saw them going, and many knew them.
They saw them departing, and observed what direction the boat took, and then hastened thither on foot, and outwent them; and so were ready to meet them again on the opposite shore when they landed. The distance by land from the place where they started would be about twenty miles.