300K is the gateway drug, as far as I'm concerned. It gives one a taste of timed distance, without destroying yourself. 400K is the one that will clue you in to whether or not it's your cup of tea. The 600K is the one where you learn if you can actually be happy on the bike for a whole day. Many choose to catch a nap in there somewhere. I can't imagine doing 250 miles, napping for a few hours, and then doing another 120. Riding straight through was my only option. One of the club guys did the P-B-P in 47 hours, and he was hallucinating towards the end. It sounds intriguing.
Anyone who rides over 111 miles in a day needs to be institutionalized, unless they are getting paid to race that distance, and even then it's marginal.