I suppose my preparation was three years of rando including three SR series and PBP. Once i decided to do a 24 hour event, I did some spin classes to step up my fitness. That seemed effective.
All that experience taught me enough to be able to ride for 24 hours, and deal with what came up. What it didn't do was teach me how fuel adequately to keep my power output constant for 24 hours. That took a few iterations.