6 months enough time to train for IM?

You have many years of cycling base, so that will benefit you.

The biggest weakness is probably that your joints/bones are not built up to running, despite your cardio fitness from the cycling. 

A run focus is probably good, as is getting up the long distance rides on the bike.

The best advice that I can give would be to make sure to not overdo the run training.  You want to make it to race day without accumulated injuries from trying to push your running too far too fast.  Since you have nearly a year, you can probably do it fine, just take it slow and don't increase your run distances/durations too fast.

Also, working with a coach would probably be one of the best investments you can make, as they can help with allowing you to squeeze the most out of the next year without incurring any injuries.

 Good luck! 

I would agree with Shermbelle.  Best if you start incorporating running ASAP.  Obviously, your legs are going to be tired during the marathon so you want to build their durability as much as possible before then.  The only way to really do this is to run often.  You certainly don't want  to risk injury by over-doing this off the bat as coredump notes, but the longer you have to gradually build-up the less risk you'll actually be taking in that department.

My guess is that if you really want to focus on "comfortably" finishing the IM, you'll probably have to sacrifice a little bit of the cycling prep you'd like to do for those early season races.  Of course, this is where your individual priorities come into play.  As long as you learn to swim the 2.4 miles, you'll have little problem finishing the bike with plenty of time on the clock to get through 26.2 on the run.  How "comfortable" you'll be during that 26.2 will be determined by your pacing during the race and your preperation before it.

Seems like it can be done since you have a bike base, you may just have to plan to walk some of the run - which is not uncommon so don't sweat it.  Better to plan to do this than overdo your run training and get an injury (I speak from experience). 

My advice since you have 6 months is to pick a training plan and get started.  I think there is a 20-week IM plan available on this site. 

Good luck!