I am toying with the idea of becoming a super randonneur next year to mark my 60th birthday. For those of you who don't know what that is, it is completing a series of audax rides in one season comprising 200, 300, 400 and 600 km efforts. I would do a local 600 and snatch a few hours sleep at home after the first day of riding so I would count that as two rides, even though under audax rules it would be seen as one.
Unless I found the experience wonderful, I would probably make that my only SR series because I am more interested in doing long distances quite quickly than superlong distances quite slowly. Ultimately, my longest ride ever would probably be a shade over 400 kms but if I ever got fit enough (and mad enough!) to do a vaguely respectable 24 hour TT I would have a go at one, maybe informally with a few CycleChat members?