Roger Bannister broke the 4-minute mile in under four minutes of training

On 6 May 1954, Roger Bannister ran the mile in 3 minutes 59.4 seconds at Oxford's Iffley Road track, becoming the first person to break the 4-minute barrier — a mark that had been considered physiologically impossible. Bannister was a full-time medical student who trained for just 30 minutes a day. Within 46 days of his record, Australian runner John Landy had broken it too; within three years, 16 runners had. The barrier was psychological as much as physical — once one person proved it possible, others followed immediately.

Source: Bannister, The First Four Minutes (1955)