I just thought of one other thing not mentioned in this string. Preparing for your first 100-mile cycling event is much like preparing for your first running marathon. You'll train for the event, but probably not go the full distance until the event. I recommend that you actually do one or two 100-mile rides PRIOR to participating in your HNH event. Cycling is so much easier on the body that doing sample pre-rides does not wipe you out like doing an actual 26-mile run. So definitely do one or two 100-milers on your own terms (preferably in a riding group of friends) so you will not be too psyched out when actually riding the HNH.
In 2017 I took up randonneuring, that's bike riding that involves events as long as 750 miles and longer. The event distance that had me a little psyched out was the 400k event (250 miles). For me it was one thing to do a ride longer than 100 miles. Not a big deal. I did not have a problem at all with that. But when I was expecting myself to ride 250 miles in a day, then I was a little freaked out THE FIRST TIME. And there was no reasonable way to do a "test ride" for a 250-mile ride. That one is more like the 26-mile running marathon example I mention above.