For my Ironman I used the following program:
http://www.trifuel.com/triathlon/ironman-workouts/
They also have a half iron training program.

I trained for 36 weeks for mine. I had done several marathons, but had never done a triathlon when I signed up for the IM. I wanted to go big or go home. I went big.

M-dot races you have to sign up for a year in advance. I find that very difficult to commit to something a year out. Mine was a smaller race, with all the frills of the M-dots- and a much cheaper price tag. And I signed up two months out when I was certain my training was strong and solid enough to complete the distance.

Have you thought about what race you might like to do?