My club has quite a few riders who have completed multiple PBPs, so there is definitely something about the event that makes them keep going back. It is a serious undertaking, so I would aim to get a couple of seasons of Audaxes under your belt to learn about riding long distances and find out what works best for you.
Best idea is to get involved with Audax UK, ride some events and chat to people to find out more. I have not ridden any Audaxes for a few years now, but have always found Audax people to be a very unpretentious, friendly bunch. It’s a different scene from the modern sportive world – fewer pretend pros on top-end carbon race blades. As a generalisation, Audax riders are more old-school, hard core mile eaters, with a pragmatic, tried and trusted approach to cycling and equipment (because you have to be self sufficient and have reliable stuff that will keep you going for hundreds of km).
Great event to aim for – good luck!