For that kind of event, I think some distance riding may be called for. Unfortunately, it takes time, a "commodity" that most people don't have. If you are young and in good shape, you can do the event, and feel wasted at the end. Young bodies recover quickly. For prep, try a century and see how you feel, i.e. can you ride the next day? If not, you have to get into a regimen of doing some 60-80s five or six days a week. Or try 250-300 in three days. If you can do this, then 335 in an event is doable.