I think that, for me, crossing the 100 mile ride 'barrier' a few years ago was quite significant.
Once your legs can keep pedalling for 100 miles then they will probably be able to keep pedalling for as long as your brain can hang in there.
I totally agree about it being a mental game on long rides. I used AA Route planner to see how far 600km actually is. As it happens it's the same as cycling from Havant, on the South coast near Portsmouth, to Lockerbie in South West Scotland.
There is NO WAY I would attempt such a thing. That would be crazy right?
However, by breaking it up into small sections (ie. a route sheet) and treating it as several 60km rides to food controls, it turns out that I actually did ride that far in under 2 days. You can always ride another 20km/1hour no matter how tired you feel but there's no way you can ride from Havant to Lockerbie.
When I'm in a bad patch I just tell myself that the next 20km is the same as my feeble little lunchtime ride and it doesn't seem so bad.
Just a mental game.
Edit. I'm started to process of getting my shoulder/neck cramps fixed (Doctor then Physio). If I can sort that pain out then Audax will be less of a mental game and more fun. Most of my longer rides have been about dealing with a "knife shoved in my shoulder blade" rather than turning the pedals. It must be nice to be able to concentrate on the scenery instead. I'm certainly not doing PBP unless it's fixed.