there is no corect way to train for the 24.
Plenty of incorrect ways though!
Roy Cormack, who held the Brittish record from 1969 to 1997 used to train by riding 25 miles flat out twice a day. Once in the morning and again in the afternoon. He only rode one or two long rides, which were about 200 miles, as part of his training.
Speed is very important for a good 24 hour ride.
I think that miles are good and help you learn how to get comfortable and how to ride for 24 hours.
But all the big hitters in the 24s are always fast at the shorter events too.
Andy Wilkinson (current record holder) held every Brittish record from 50 miles to 24 hours all at the same time.
Gethin Butler came from a background of racing.
If it was all about doing the miles, then I would do much better than I do now. The distance is no issue for me, but the speed is. But if I've done the miles and am not too tired at the start, I am usualy much faster than my closest matched riders on the finishing circuit. Miles are very good for your strength, but not great for your speed.
It's a matter of finding the balance. My scales are tipped heavily in favour of strength, but my speed is very poor. (PB for a 24 is 443, PB for a 25 is 1:03, but that was 16 years ago and I've lost some speed since then and I haven't ridden a TT shorter than 12 hours since 1995, except for a hill climb)