Despite illnesses that have diminished his once powerful voice and left him frail, the Rev. Billy Graham vowed to preach each night of his final American crusade. He kept that promise Friday night, the start of a three-day event at a city park that marks the completion of his six decades of mass meetings. Before a crowd of about 60,000, Graham took the stage using a walker and leaning on his son and successor, the Rev. Franklin Graham, who stayed nearby in case his father was unable to go on.