what he said
Agreed.
When I was still do a heck of a lot of training for tri, rule of thumb was don't train to the distance, train near it, as the others have mentioned, and also try and put in a smaller ride in the same week, each week, so your body gets used to the whole idea of the distance, even if it's broken in two as such. This will make it easier to say do your 100 mile ride. Watch your nutrition and hydration too.