Option 3 - ask the organizer what to do!
We have 2 local clubs, a "roadie" club and a "casual" club - both of them welcome newbies and assign someone to ride with them and help them out. The roadie club, though, only does this once, then you are expected to ride your level on your own. Still, they are supportive.
The casual club would not hesitate to welcome a newbie and carry them along for awhile.
So, find out what this group expects - are you expected to keep up? Do they all ride at their own pace then meet up along the way or at the end (as the local Roadie club does)? Do they have people willing to ride along with you and mentor you (as the local casual club does)?
A paceline is pretty serious business.
If you've done 1/2 the distance or more, I wouldn't worry about the distance, might be challenging but you can do it - but I would worry about the pace and expectations - a first time long distance requires you to pace yourself for the long haul, not for speed.