Reading between the lines or race entry list is the reason for them really so that you think you are mentally prepared because in training you have done
2.4 mile ow swim. Tick
biked over 100 miles. Tick
ran a marathon. Tick
your mind now says that since I have done the relative distances ,I know I can do all the distances I have increased the probability of succeeding in the IM.Your mind is full of perfect planning prevents piss poor performance etc.
Providing you have respect for the race and can adapt to changes then you should have no worries.
One of the best preps for the distance is to have some real nightmares of races and get over the disasters thereby mentally helping,rather than lulling you into the false sense of security that everything will go to plan.
As a rule of thumb the max distance when I used to do marathons on their own that I would do in training was a 22 mile run,followed by a weeks rest then a couple of weeks of tapering prior to the race itself.That was good for a 3.15 marathon on a hilly course,no need for a run of 26.2 miles.Anything over 16 miles and it will have a detrimental effect to your body.
The only time I cycle over 90 miles is during an IM.
If you really need to do 26.2 miles,could I suggest you do 1 mile run 1 mile fast walk,it will reduce the impact on your body and only take about 5 and a half hours.