I've done a pretty fair number of 1000/24 rides and one 1500/24 (Never again, thank you very much
) I think the best way to prepare for an "Ironbutt" style of ride is to begin riding bigger miles. If you can ride tank to tank for two or three tanks you've pretty much got the endurance part down.
A certain level of core endurance is good as well. For me, working out helps me ride longer distances, it might for you as well.
Remember as others have said, do this for fun. For me, it IS fun. I live in the West, and watching the miles unspool from the seat of a powerful motorcycle is soothing and liberating to me. It isn't boring, and I certainly agree with others who say, "Don't do this for bragging rights..." Ok, maybe just a little bragging is ok...
Enjoy, take some pix, and let us know how you did!