Congratulations on taking the plunge and signing up for an Ironman triathlon. The registration usually occurs a full 12 months before the event so you have a lot of time to think about it and prepare. My coaching starts in November but I always like to share some tips for athletes in terms of what to do between now and then.
Firstly, try not to think about it too much! Training for an Ironman takes up A LOT of mental energy. Athletes understand it requires a lot of training but many don't anticipate how it can dominate your thoughts too. If you didn't already have the Ironman talk with your loved ones then now is a good time. You need their support and understanding. When the real training begins in the new year you will be at times busy, at times tired, and at times boring (because all you talk about is triathlon!)
I will now share with you one of the greatest gifts every given me. A friend was in the late stages of training for an Ironman and said to me: "I'm in the best shape of my life but my life is not in very good shape." Plan for your journey and hopefully you can keep your life in good shape.
In the summer before your IM it's a very good idea to gain some experience in the sports you may not have participated in as much. To be clear you don't have to do this, it will just help make things a little easier down the road.
The Swim
If you haven't done long swims then consider building your swim to at least 3k if not 3800. Come fall it's time to get in the pool, ideally as part of a coached group, and work on your technique and efficiency. You want to complete the swim of the IM in a solid time but more importantly without wasting energy doing so.
The Bike
If you haven't done long bike rides then try to build to at least build to 100k. If you have the time then going a little beyond that will be very beneficial since a majority of your training time will be on the bike. This gives you some time to figure out your equipment and your nutrition. On long rides try to take in 50-60g of carbohydrate per hour. This fueling is one of the most critical aspects of success in Ironman.
The Run
You don't have to have run a marathon before doing an Ironman. That said, if you haven't run one there is a benefit. A fall marathon is a good idea if it works for your schedule. I do not recommend a spring marathon preceding the IM in summer. You will lose too much training time to the marathon preparation and recovery. Athletes do it and succeed. I just don't feel it's worth it. A fall marathon on the other hand will boost your aerobic and biomechanical fitness then allow for plenty of recovery before the weather turns cold and the bike prep really begins.
Triathlon
And of course do a triathlon! It doesn't matter the distance but an Olympic or Half-Iron would be great. This will simply give you event day experience with the nerves, logistics, pacing, and more. And besides, it will be fun!
Strength and Mobility
Triathletes are notoriously bad at making room for strength and mobility training in their week. Change that. Find the time. Not only will it improve performance it will also help you to avoid injuries.