First time triathlete.....first time ironman......worried.

The key for you will be to add volume and speed in your training without overloading your body and getting injured.  One thing I would caution is to have realistic expectations.  Stress your body such that you're building the aerobic base to perform the distances.  If you try and do too much speedwork to get faster you may be inviting injury. Athletes who are incorporating speed and racing at this distance have YEARS of base work under their belts.  

Stick to the plan.  There's no magic in this thing, just consistent, hard work.  If you put in the consistent work to build your base you will be surprised at the speed gains you will have from just doing the work (and losing some weight).  Adding sessions may only serve to invite injury.  With this being your first year in multisport, you don't know how your body will necessarily respond. See how the plan goes.

In the back of the Fink book, he lines out the paces you need to hit to "just finish" How fast are you cycling right now at a pace that you can (or think you can) sustain over the 112 miles? Bear in mind your run pace will be after the swim/bike so probably slower than 11:30/mile.

Be sure to rest and recover well to avoid injury.  Good luck!

Without a clue of your current training.. I'd agree that you must follow your plan religiously. One can't cut corners with endurance. Especially the run portion.

If you can, you might want to get a swim coach for a few lessons. 1hour 55 min will cut it way close and I think you can increase your time on that if you are able to focus on improving your form. Having a better swim will help you get through the rest. DO NOT underestimate the importance of that portion of the training. 

Good luck in your training. 

If at all possible, I recommend signing up for at least 1 shorter triathlon before your full.  Open water preferably. 

Completing an entire triathlon is a totally different experience than training.  It will give you a chance to run through everything, and experience an OW swim with others around you.

If you are disciplined to follow a coach and their training plan, you only need one other thing in my opinion...a mindset on race day to be flexible. Flexible to handle what doesn't go your way...and find a way to keep going regardless.

Flexible after the swim means accepting your swim time and going to the bike leg without acting defeated.

Flexible in the bike means constantly managing your effort against your hydration and nutrition. Which your long rides during training would have helped hone.

Flexible after the bike means mentally leaving the bike leg behind you and adopting a positive attitude towards the run.

Flexible in the run means managing your effort against your hydration, nutrition and the clock. Better to balance and finish in 16:57 then push hard in the first 6 miles, blow up, and DNF. Manage the first 20 miles so you can finish the last 6.2.

I finished 4 IM's last year in 4 weeks without injury. Trust your coach. Become an expert on your personal endurance engine. Do EVERY LONG WORKOUT in training!!! Then be flexible on race day. Even pro's walk during the run in an IM!

I will offer a contrary position.  An iron distance triathlon is very hard on the body, and 17 hours of aerobic activity when you are not trained for it, could bring serious repercussions.  An open water swim in an ironman is challenging for even experienced swimmers, and you admit to your limitations.

You will find encouragement on this board, but do not let it substitute for good judgement.  You are ultimately responsible for the decision to compete in an ironman distance race, and you are being honest with yourself, you are not ready.  Honor your Mum by raising money in other shorter races with the goal of getting yourself in condition to confidently finish an ironman. 

^^^This is very good advice, it will make a world of difference.

Condolences on your mum but you may want to give this some serious thought as not the best idea. There is so many things that go into completing a full ironman that without any previous experience with shorter races and a longer preparation period to handle the bodily stress you will encounter you stand a good chance of not only not finishing but worst case ending up on the side of the road in a pile or on a gourney in the med tent. If you truly want to do this take the time to do smaller races and build up your endurance over time. I'm not saying its impossible to do and I'm sure there have been plenty that have done it without much training but I guarantee you it wasn't a pleasant experience for them. I will have 2 solid years of training and racing shorter to longer distances building to attempt my first Full Ironman. This will not only give me the best chance I will finish but also have a great experience. Good luck with your decision either way.

The OP is 16 months old...