training depends on what distance you want to do…
Super Sprint
Sprint
Olympic/standard
Middle distance (also referred to as half ironman)
Long distance (also referred to as ironman)
(ironman is just the brand which runs some races of those distances)
I mainly do Olympic distance, though may do a half at the end of the year.
Swim training is mixed, some sessions I’ll do upto 2.5km probably, others are more like 1km total distance. Vary it between steady swimming (once a week tops probably) and the rest as intervals to increase speed & speed endurance. You’ll generally find that the ability to swim longer at a steady pace comes with increasing the amount of reps & diatance per rep when doing intervals rather than needing to do lots of steady long swimming.
There’s some good books, Joe Friels ‘Triathletes training bible’ is a great place to start!
EDIT – and swimming is all about technique. Get that right and strength in the right places will come. Get it wrong and you’ll get strong arms but you wont go very quickly… get it right and you;ll get strong shoulders and glide along nicely at a surpsing speed with minimal effort. Always worth getting lessons for technique early on as it has such an impact. either through a tri club or from lessons at your local pool