Jamieson, Fausset and Brown on Numbers 10:33

33. they departed ... three days' journey--the first day's progress being very small, about eighteen or twenty miles.

ark of the covenant of the Lord went before them--It was carried in the center, and hence some eminent commentators think the passage should be rendered, "the ark went in their presence," the cloud above upon it being conspicuous in their eyes. But it is probable that the cloudy pillar, which, while stationary, rested upon the ark, preceded them in the march--as, when in motion at one time (Ex 14:19) it is expressly said to have shifted its place.