It obviously refers to a trotter's mile rate. I believe the song was written in 1850. At that time a trotter capable of covering a mile in 2 minutes and 40 seconds was stepping lively. I believe the current world record is 1:49-3/5 on a one-turn mile racetrack. Lady Suffolk, the "Old Grey Mare" in the song was the first horse to break 2:30 for the mile in the late 1830s or early 1840s.