candy's IM training mantra still sticks in my head and is my method
swim a bit, bike a lot, run a lot
you an get over-anal in training and provided you put the load in and have some reasonable chance of making the cut-offs then an IM is doable - but there are many other factors that play a big part - nutrition, health, course, weather etc...
if you enjoy training to plans - fine; if not - just do lots of training and work on your weaknesses