Hi,

If you want to do 100 miles and enjoy it, and are not that
bothered about the pace you do, then the basic idea is to
to just rack up average mileage without taking it too hard.

I imagine cycling is the same as distance running, where you
try to get to the point 1/3 distance at planned pace is a walk
in the park, and most of your training is up to 1/3 distance,
working on the pace that is "a walk in the park".

Occasionally 1/2, but one or two long rides beforehand
won't help at all, as long as you expect to be knackered
at the end of the 100 miles, all they will do is knacker
you somewhat less, and you'll need days to recover.

rgds, sreten.