I fuel like in a race - around 50-60 grams pr hour. I run loops or have my girlfriend on a bike with bottles :)

I eat full breakfast before and I don’t try to limit carbs, as I think my limiter is my muscular Endurance and not my ability to ‘turn over energy’ <—- sorry I don’t know the english explanation/words

I'd further argue that your IM bike ride should be one of your easier long rides of those last 6-8 weeks of training.

Is it just the RPE difference between being tapered vs in-block training fatigue? Or is is because those long rides in IM training should be down above IM effort (like you're example of including 70.3 efforts into the long ride)?

To make sure - try to swim hard in the morning, bike 5 hours including 4x50 minutes @ racepace and then run your long run the day after. How did the run feel? If it takes more than 4-5k to feel "normal," then you went too hard on the bike the day before (or didn't fuel enough during or after the bike).

I did a "fun" experiment at IMTX this year. The context was that I was training for the Comrades Marathon (89km ultra road race in South Africa), which has about 20,000 participants. Your start corral is based on your qualifying marathon time, with the front one (the A group) requiring a sub 3 open marathon. However, you can also qualify with an Ironman run time, which for the same A group qualification, only requires a sub 3:30 run. I did my winter training block, with everything on target for a sub 3 at the Houston marathon. However in the race I had a super weird issue with quads totally seizing at mile 11. I walked the rest of the marathon, since I had to at least finish one marathon to qualify for Comrades at all. It was something like a 4:25 which basically had me in a really bad starting batch for Comrades. So I had to do another marathon if I wanted a better time. I then did the LA marathon, aiming for just a decent qualification but not the best, conservatively shooting for a 3:30. I ended up doing 3:31 or something, so bettering my starting position but still not the best. I was holding back a little bit in that race because I didn't want to blow up again but I was totally spent at the end. Maybe at best I had a 3:20 in me that day.

So then I thought, what if I do IMTX, and just sandbag the whole race up until the run and then go for it. So I swam super easy, soft pedaled the bike, spent 20 minutes getting ready in T2 and then went for it on the run. I ended up with a 3:28, which although I've run faster than that before, was about the same as my best open marathon at that point in time. Of course, I gave up a lot of time on the bike (probably 40+ minutes at a guess) so this would be a really poor strategy to get a good IM time, but it does provide the extreme end of the spectrum for what happens if you save yourself on the bike. For what it's worth, my best overall IM times have been off really hard bike rides. It just becomes a gamble as to whether or not you blow up on the run. For me, the main factor in blowing up (or not) on the run was always related to the bike hydration, heat management or nutrition, not the bike power output itself in isolation.

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