HIM and Marathon Training

I wouldn't do an October half ironman followed by a November marathon.

Are you trying to maintain your cycling and swimming base during marathon training? A beginner plan may help you out. It's likely going to be a little exhausting trying to maintain a base in the other events while training for a marathon.

You'll likely be building your fitness through marathon training, so I wouldn't worry too much about the swim and bike. Lots of plans have day where it's a short run or cross training. Just sub in some biking/swimming on the cross training days. 

Also, there are plans that build from a half marathon to a full marathon. These might be a good place to start, just make sure the run volume at the beginning is in line with what you were doing for the half iron.

Training for Vineman will give you a great aerobic base.  Problem with having a strong aerobic base tho', is you transfer that to running;  and your heart and lungs will write a check that your bones, joints, ligaments and tendons can't cash.  So, I'd be doing a run-focussed HIM training, that incorporates 4 days/wk of running.  So, even while you're building your cycling and swimming, you need to make sure you're building a run base of 25-30mpw.  (or more)

After Vineman, you'll need a week or two of light recovery workouts, and then start in on a full marathon training plan.  Another HIM in October is probably not a good idea- unless you already have a strong base and you don't intend to go all out.

If this is your first marathon, you'll find that the training for it (if you plan to 'rock it' and not just 'finish' it), is pretty serious.  You'll be running 5-6 days a week and doing 40-50mi weeks (or more).  There will be a couple days a week where you can cross train on the bike and the pool, which will certainly help out- but you will lose bike/swim fitness in favor of run fitness.  There's really no way around it.

Maintaining fitness is like blowing up a balloon with a hole in it.

Tri fitness, is like blowing up three balloons each with holes in them.  When you blow in the bike balloon, the air is coming out of the run balloon.  And that's life. 

I like that analogy.

Also, just wanted to add, there is nothing wrong with a single-sport focus for a while. In fact, it can be extremely beneficial!