I think most randonneurs would agree that the reason to do rides longer than 5-6 hours isn't to build endurance, it's to learn what you might need at that point -- does your bike really fit you well enough? Not about breaking your butt in at that point, but whether you've got the right saddle, for example. Or finding out what your digestion does after 10 hours of riding.
But people also say to riders at the end of their first 200k "you finished the 200k and don't look like death warmed over, you'll be fine on the 300k". If you felt great at the end of some 100milers with similar climbing profiles to the 312 (I'm not familiar with it but was under the impression that Mallorca is hilly?), you probably know some or all of those lessons, but there might be a few more that would be good to know ahead of it, especially if this is a big destination ride and you're traveling for it and will be really sad if you don't finish.