Since the earliest times of the Church, there is evidence of some kind of penitential preparation for Pascha (Easter). For instance, St. Irenaeus (d. 203) wrote to Pope St. Victor I, commenting on the celebration of Easter and the differences between the lenten practices in the East and the West.
After the legalization of Christianity in 313 AD, the disciplines of Lent became more regularized. The Council of Nicea (325 AD), in its disciplinary canons, noted that a synod should be held each year “before the forty days of Lent.”
St. Athanasius (d. 373) in his Festal Letters implored his congregation to make a forty day fast prior to the more intense fasting of Holy Week. St. Cyril of Jerusalem (d. 386) in his Catechectical Lectures wrote eighteen pre-baptismal instructions for catechumens during Lent. St. Cyril of Alexandria (d. 444) in his series of Festal Letters also noted the practices and duration of Lent, emphasizing the forty day period of fasting. Finally, Pope St. Leo (d. 461) preached that the faithful must “fulfill with their fasts the Apostolic institution of the forty days,” again noting the apostolic origins of Lent.
These references lead one to a reasonably certain conclusion that by the end of the fourth century, the forty day period of Easter preparation known as the “Great Fast” in the East and “Lent” in the West existed, and that prayer and fasting constituted its primary spiritual exercises.