best way for iron man training

There is no such thing as a 12 week Ironman training plan, at least not for beginners.  Most beginner plans are 24-30 weeks long.

Most people ease into IM distance by finishing shorter distance triathlons first.  Olympics and half ironman distance.  A lot of people do half marathons and some full marathons to improve their running (although this is not a requirement in any way).  Like you did, a bunch of people do centuries on the bike.

The standard most-recommended book for long distance triathlon is "Going Long".  Buy it and read it.

http://www.amazon.com/Going-Long-Triathlons-Challenge-Multisport/dp/1934030066

But for you, I'd recommend "Be Iron Fit", another super popular book.

http://www.amazon.com/Be-Iron-Fit-Time-Efficient-Training/dp/1599218577/ref=pd_sim_sbs_14_1?ie=UTF8&refRID=0XQ9MY7DJ8V71FMEG8GF

Read both.  Going Long is a little older, Be Iron Fit is more "you're busy, you want to do an IM, here's how".  Be Iron Fit talks about their "just finish" training plan, which would probably be good for you based on the intensity you probably want to train at to avoid injury.

What is your current fitness background (in general, and for swim, bike, and run)?  How old are you, man or woman, busy family, rich single adult, what?  how much time do you want to commit, how soon do you want to do your IM?  All these things matter.

sounds like you're on the right track, you don't want to start training a month before the race...so kudos

First and foremost, the key is consistency.  if you can get in consistent time in all 3 sports, and just keep at it, you will be fine.  Near the end you do have to throw in some century rides and long long runs.  For the most part though, its about NOT overdoing the big workouts, and just plugging away.