Once you get to the Half Ironman / Ironman level, I would encourage everyone to work with a professional coach to ensure that you develop at the appropriate pace and reduce the likelihood of injury and burnout. I work with a fabulous coach Brian Rabinowitz who is a finisher of multiple Ironman events himself.
You can download a sample of my current program SL 16.01.12. You will note that everything is time based and we generally work on a 2 week on, 1 week recovery cycle.
If you want more information please feel free to contact me directly.