I like 24/12 fwiw. From vague memory, your solo issues are more mental/reflux related than fitness? Have you ever finished a 24-hour solo?
Edit: I guess what I’m saying is that in your position, I’d be looking at the other factors which have maybe been an issue in the past for you and addressing them in advance – diet (reflux prevention), you had back pain no, somewhere I bumped into you a few years back (bike set-up?) etc, mental approach etc. I’d have thought a lot of road-based fitness would be fine. Says me as a rubbish soloist 😉
Oh, and the nice thing with 24/12 is the course is generally fun to ride repeatedly, especially last year when it ventured up onto the moors, and there’s a cracking atmosphere, plus the location – touch wood – seems to mean it’s less prone to collapse into a mud fest.