Whilst I agree the winners are pretty extreme I'd imagine (which is all I can do having only read the race reports) that there are more "relaxed" riders that participate in the Iron Butt.
Think marathon winners running at 2 to 3 hours to the others running at 4 to 5 (and yet others running a lot slower).
That said, if you've never even tried some long distance riding there's a few things to try..
Try the saddlesore..but also try 1500 miles in 3 days - or 3000 in 6 days...I'm hesitant to suggest more until you've tried at least that. I think you'll be surprised at how hard you might find it. If you manage that then try some of the other IB challenges and if you're stilll on board go for the rally.