Periodisation training can be of great help for an Ironman Triathlete. After a race season you should plan your next season by dividing your training time into the following segments.
Recovery- This should be complete rest then very easy training. Riding solo or on an indoor bike keeping your heart rate under 65%.
Endurance- Building up consistency and the length of each weekly long session then building the duration of all other workouts. Spend a lot of time easy building up time training.
Tempo- Upping the tempo with once a week tougher sessions on hills into a head wind riding off road. The aim is to build up strength endurance not speed. Medium even paced efforts can also be done on the turbo. Duration Swimming 10-45 minutes cycling 30 minutes to 2 hours running 20- 75 minutes.
Threshold- this effort needs a thorough warm up and is at 60 minute flat out intensity. Swimming can be 400-1500m, cycling for 5-15 minute segments, running 4-15 minute intervals. Maintain these for 4-6 weeks until you are fit enough to do longer sessions.
Anaerobic training and V02 max sessions- Hard enough so you can only maintain this for 3-6 minutes. Once these have been completed for 4-7 weeks incorporate flat out 30 seconds to 90 seconds. Bets done on very quiet roads or an indoor turbo.Sprints, hills, with a faster training mate.