Great read, 515mm. Spookily, I had exactly the same experience. This year I was determined to crack the Gran in under 8 hrs. Only a modest 15mph ffs. I foolishly thought that by stuffing enough fuel in the engine, bananas, bars, cake and crisps, and keeping hydrated with water and salt levels up with electrolyte I would do it easily. I added a couple of anti-cramp pills to one bottle which seemed to make me want to pee. I completely forgot about pacing.
It wasn't until just after halfway coming up to Crai in that headwind, with no group in sight let alone touching distance, I realised that I was hardly turning the cranks. At Cimla I was standing up at the urinal when I had cramp attacks in both legs simultaneously (I don't recommend this). I had to stop on the second Bwlch climb twice just to gather myself, but once over the top I was hammering it all the way home. Needless to say I failed to reach my target by a fair margin!