Convert from military pace [double time] to mark twain

Unit Definition (military pace [double time])
The length of a single step in marching (taken to be 30 inches for quick time or 36 inches for double time).

Unit Definition (mark twain)
The mark twain or twain is an old word for the number two, derived from the Anglo-Saxon twegen. The American author Samuel Clemens (1835-1910), who had been a riverboat pilot on the Mississippi in his youth, took his literary name from a traditional riverboat phrase "mark twain", meaning "exactly two" fathoms of water. This was the minimum depth needed for the boats to operate safely without running aground.