Triathletes talk (a lot!) about swim-bike-run training. That can depend on coaching philosophy and individual training protocol. There is no one set periodization that should be followed for all individuals. A big mistake I see a lot of athletes (particularly with the bitter winters of the north east) do is scale back over the winter and arrive in April with a blank slate, scrambling to get fit for their first race in June. The problem with this is they have no foundation or groundwork to work upon to increase fitness safely without getting injured. April is too late to try and build a base to gain fitness from, work on skill development and spend time getting their race fueling protocol, bike fit, run shoes, wetsuit etc. in racing form.