Origin and history of training

mid-15c., "protraction, delay" (a sense now obsolete), verbal noun from train (v.) "drag along the ground."

It is attested from 1540s as "discipline and instruction to develop powers or skills" (see the verb for sense evolution); specifically by 1570s as "military drill" (hence training day, 1670s, appointed for drill and review of the citizen militias). It is attested by 1757 as "practical education in some art, handicraft, etc. by instruction combined with practice."

In the athletic sense it is attested by 1786 as "exercise to improve bodily vigor, act or practice of developing physical strength and powers of endurance."

Training is the development of the mind or character or both, or some faculty, at some length, by exercise, as a soldier is trained or drilled. Discipline is essentially the same as training, but more severe. [Century Dictionary]

Training wheels as an attachment to a bicycle is from 1953.