What down implies when used as a preposition in a sentence
(1) We use 'at' to describe the speed of something - People can move at a slow pace, at a jog, at a run, at a furious pace. These people ran at a jog. He drove at 60 kph. (2) 'Up' and 'down' are to be considered relatively. They ran along one street in a certain direction, and then moved to another (presumably adjacent) street and ran along it in the opposite direction (probably arriving not too far from their starting point). Up and down are opposites, as I expect you already know. In constructions of this kind, 'up' is usually first, and 'down' second.