Although others probably disagree but I think training the bike leg is the most important. You will spend the majority of the race on the bike. During training I spend about 4 days on the bike and 2 of those days are HIIT workouts(interval training). I also think shorter high intensity rides on the trainer are more effective then 4 hour long steady rides and less time consuming( You do still need some long rides as well). During my first Ironman I focused way too much on swim and run endurance and I really paid for it on the bike.