I'm sure you'll do fine on a 600k. All you have to do is keep going. "After 200 miles, nothing changes." :-) Just keep telling yourself that. Sure it's a really long day, followed by a long day. But if everything is all right, bike fit, nutrition, electrolytes, etc., and you have sufficient base, which I would consider the ability to ride 200 miles and not die, and the ability to ride 100 miles as a fun training ride, then you should be ok.

Even if you don't have someone to do 508 with, chances are you'll fall in with someone. I almost always do. For the later parts when it matters most, anyway.

Nanci