I see that the sun has brought out the planning stage on the Audax thread -
http://citycyclingedinburgh.info/bbpress/topic.php?id=15383&page=3#post-212752
I'm sure that there are many people on here who regard riding 100s of kms as 'impossible' - or perhaps an 'aspiration'.
Many more may think closer to 50 (km or miles) is more realistic for them.
Perhaps this thread could collect suggestions with that sort of mileage either as loops or one way with a train journey for the first or return part (or for both).