How far are you cycling at the moment? Or how far can you cycle comfortably at the moment? If you can do 10 miles, then next weekend, do 10 in one direction, stop for a coffee and cake, then 10 back. Then another 10 on Sunday. Following weekend, 30 with a coffee break. then 40 with a coffee break.
Then the sportive. You'll be fine.
Edit: Just had a look. You have 10 hours to finish. If you can comfortably cycle 10 miles, I'd relax. I've taken people on 60 mile cycles when the longest then had done was 10 miles. Just broken it up into 6x10 mile rides with a good break over some food. You are nearly fresh again after the break. If you cycle 10 miles at 10 mph, then you can sit for a full hour before moving again.