This use of “a-” forms adverbs and prepositions and comes from the “on – ” preposition which we saw in “ablaze”, so both Prefix 3 and Preposition 1 are in play here!

• 01.109 and three may walk abreast” say the runes,
• 03.040 and three may walk abreast.'”
• 13.048 wide enough for many men abreast.

“a-, prefix3.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.

“a, prep.1.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“abreast, adv. and prep.” OED Online. Oxford University Press, March 2015. Web. 16 May 2015.