Periodization
Periodization is the organisation of training into phases, each with a different focus, that build towards a target race. A common sequence for endurance athletes is base, build, peak and taper, followed by a recovery or transition period.
The phases
Base builds aerobic endurance with mostly easy volume. Build adds race-specific intensity — threshold work, race-pace sessions, bricks. Peak brings the longest and most specific sessions. The taper sheds fatigue before race day. Within each phase, training usually runs in blocks of three or four weeks with an easier recovery week at the end.
Planning a season
Work backwards from your most important race. Secondary races can be trained through or used as tune-ups. Two key races close together need a shorter rebuild between them rather than a second full cycle.