Wife and I did a 35km ride on Saturday. Sure, we were bugged by some awful town road conditions where it was safer to dismount and use the sidewalks but end-to-end was about 3 hours. We had occasional stops to take in the view and have a drink, so as a social ride, it was a success - very much so.
What I learned about this, is that I'll peg the distance we go on any new rides to no more than 25-30km unless it's an on-road charity ride for example, where 50km would be about right for us.
Another thing I'll also do if possible is make sure that the half-way bail out point is easier going back than carrying on. Just in case we need the easier ride back to the starting point because of injury, that sort of thing.