Old English myrge described things that gave pleasure, including music and the natural world. Merry later became familiar as a description of cheerful people. In a phrase about pace, it can describe brisk movement.
The pace use is an extension in the modern range, not a claim about its exact first date. A proposed deeper connection with short duration remains outside the scored trail.
Source
Etymonline: merryThe entry gives pleasant myrge applied to things and the later cheerful-person use, while qualifying the deeper short-duration connection.