I think its wise you are planning on taking it "easy" at IMAZ and testing frequently
I just did B2B full Oct 26 less than 3 months after being diagnosed. I took a very easy and relaxed approach and finished 13:21. No issues.
I am almost done with my race report. Its taken forever(5 pages) because I'm trying to be detailed on what I did nutritionally and testing in hopes it can help other diabetics in some way.
So far I've only consumed minimal carbs during training, but most of my training has been at a very relaxed pace as well. I plan to use UCAN during the race and have tested that a couple of times such as during the Horribly Hilly Hundreds 200km ride (also ate two 4oz packets of wholly guacomole,) and a 2.4 mile practice swim at a faster pace. I did a 150 mile training ride on nothing but some almonds at the halfway point and then I sucked on a date or two every hour.
I'll have my bacon and eggs for breakfast and then have 2 servings of UCAN early on the bike depending on how my body reacts to the swim. I'll carry extra UCAN in powder, and planning on another 2 servings of UCAN at halfway. I'll maintain an emergency supply of a couple of Bonk Breakers on me at all times, but I'll only use them as needed, further reducing my basal if I trend low.
Since bike to run transition causes a bump of about 30-40 points I'm hoping to target 80-90 mg/dL at the 90-100 mile mark at which I'll cancel my temp basal (currently 0.4U/hr) for about 90 minutes. Once my body begins to settle in to running (about mile 3 or 4), I'll hope to turn the temp basal back down to about 0.1U/hr, knock back more UCAN and then carry on.
This 'plan' isn't set in stone. The only way for me to know a good plan would be to have done this before. I have an outline and the plan is to adjust. During training my body has had the ability to compensate to produce or access it's own carbs whether it's the muscle glycogen, or liver glycogen from gluconeogenisis. I'm aware that it could theoretically run out of these limited reserves, it just hasn't happened to me yet, in part do to increase metabolic efficiency and all the other compensatory factors of being ketoadapted.
My longest training run was 16 miles. I consumed only water (and electrolytes) during all training runs, even on an 80-12 brick. I understand things will probably be different for the full 26 after the swim and bike. So I'll likely consume at least two more servings of UCAN on the run, and carry a bonk breaker at all times.
That's why this one is considered research. Adding in the UCAN shouldn't bother my stomach. I've had an iron stomach in the past and used to be able to play soccer on taco night with little issue. The bit that I have tested UCAN was enough to confirm that I don't have any issues with that either. I'm a proponent of not doing anything new on Ironman day, but I'm comfortable with UCAN and it's very low osmolarity shouldn't cause any issues.
I'm usually about an 11hr finisher (pre-diagnosis and one while un-diagnosed), but I'm okay if this one takes 14 or more. A relaxed approach can actually be pretty fast since it usually leads to great pacing.