None of us can put ourselves in your shoes. Do you know what the terrain is like on the organized ride compared to your usual rides? I only ask because back-to-back+ rides over terrain that involves more climbing than your used to can make for a difficult recovery. OTOH if your backside can deal with multiple days in the saddle and the terrain is very doable and you stay hydrated (what's the usual weather BTW?) you may want to give it a shot.

You seem to have the base to keep your legs moving, you should be able to get plenty of rest, nap in the afternoon and good night's sleep, so finishing wouldn't seem to be out of the realm of possibility. Think of the sense of accomplishment.