WELL, that was interesting!  (with apologies in advance to 
@ianrauk for continuing to kick the erse out of this...)
Decided to do a sort-of prep ride from home covering the start out to North Berwick and then the end back to Mussleburgh - 
here is the ride stats.  It was actually around 150k, I stopped to stretch and pressed stop on the Garmin, then forgot to restart! 
 
How do I feel?  Well the first 60k-odd was pancake flat so I made really good time out to North Berwick, start throwing in some hills and that soon slowed me up.  I need to get my pacing right as I started easy, got on a nice bit of road and started making good time but wasn't leaving anything in my legs for later.
I felt a bit sick near the end but that might be due to a) the heat and b) the bottle of Diet Coke, Wispa and Feast that I demolished back at Mussleburgh - so I need to get my eating and drinking right.
I DIDN'T get cramp which I was pleased with, though I think if I'd pushed it I would have.  I 
think I got lost from the route so will need to check that - I fixated on getting to Gifford after Haddington without reading the instructions, though it may not be a huge issue.
At the end of the ride I couldn't imagine doing another 10k never mind another 50, and I had a bit of a wobble at 100k thinking "this is only halfway!!!" so nothing is guaranteed - but I'm hoping the adrenalene and maybe some company on the day will get me through.