In reply to Pids:
There's no need to ride 100 in advance, but as others have said you need to get used to the time in the saddle. This can be a huge factor in sportives as your arms, back and shoulders can take a real hammering if you're only used to riding for 3 hours and you suddenly up it to 6 or 7.
Are your 40 mile rides with other people or solo? You need to get some experience riding in a group if you don't have any. The good news is that it is much easier in a group, so if you can ride 60 solo you should have no trouble riding 100 in a group as long as you know how to ride together.
I'd suggest riding in a group for 3-5 hours each weekend (ideally three building up to 5, but that's not really in your control unless you pay them!). Maybe drop it down to only 1 or 2 hours the weekend before the 100 miler.