PBP this year was my first event with more than 600km. For me it was a very enjoyable and undemanding ride. Almost completely pain free, finished in less than 70h etc bla bla.
In time between November 2010 and August 2011, I tried to ride 3x a week, each time for at least 1h, ideally covering at least 100km
per week in total. I reckon, I managed to stick to that regime 3 out 4 week... So yeah, riding regularly certainly helps.
Between November 2010 and August 2011, my total mileage was a bit less than 4800km, ie less than 4x the distance of PBP in 10 months - not too much then.
I believe what made PBP so easy although my low mileage was the fact that I rode nearly all my training miles and the complete qualifying SR series on a fixed gear bike. But I rode PBP on a geared bike. Hence my advice would be - train on fixed, ride with gears. Anyway riding a fixed wheel is good fun...
Regrading increasing your max distance remember the
first golden rule of cycling insanity, you can always cycle at least twice the previous greatest distance you have ever done.
One last thing that helped me a lot - train your back to prevent a stiff neck. I did the 100 push-ups challenge in the month before PBP. It works! Stopped with the push-ups after PBP and now I can feel my back again after last night's 40k pub run...