In reply to WillBroad:
Easy, I did a half with 5 weeks swim and run training on the back of being bike fit. (I have also subsequently done full)
You clearly have base fitness if you are doing a 3.30 marathon.
The swim isn't nearly as difficult as it sounds as you are jet propelled in a wetsuit.
4 months is plenty of time to be able to do 112m on the bike without too much trouble.
Get a good bike though, I can't stress this enough.
Nutrition is key.
And I predict sub 12hours for you
(1.20 swim, 6.30 bike and 4hr run)