Thanks for your concerns on the distance. I'm pretty sure I know what I'm letting myself in for.
I did a middle distance tri last weekend, the bike course was a rolling 51 miles and I managed to hold on to 21mph. I felt like I could have gone on. That 51 miles was on my TT bike as you say, on the tribars and on the nose of the saddle. The bike is a proper TT bike, by which I mean that it's got a very steep seat angle making a low position comfortable.
I ride my TT bike to work lots so I'm not worried about holding that position for 100 miles (or more).
The 100 mile TT is part of my training for an Ironman in September. I hope that explains why I'm not interested in 10s or 25s much. I'm not interested in absolute top-end speed. I'm interested in keeping my legs doing 20-22mph for hours on end without getting too tired to run. The primary reason for doing a 100 is to get nutrition right.
Speed training is very good for your endurance.
Roy Cormack rode 25 miles at race pace every morning and every evening as training for the National 24 hour Championships in 1969. He only rode over 200 miles once or twice that year, I believe.
His record 509 mile ride stood until 1997, when it was broken by Andy Wilkinson (525 miles) with new technology (Carbon fibre bike and tri bars)
Andy didn't think he would do it on the day. It wasn't a good day (windy, sometimes wet) as he'd been doing a lot of track racing, so didn't have as many miles in his legs as he usualy did.
All the best 24 hour riders pay a lot of attention to their speed training. Admittadly most get the miles in too, but too many miles can slow you down. You should be OK with a 100 though, if you recover from it properly.