Short version - I returned from Scotland in 2013 with the power of about one & a half legs.
It seems I damaged my right knee & surrounding musculature trying to hold onto a train that was a fraction too 'hot' for me.
But I'd masked the pain with 'vitamin I'
As a result, from Brampton to Gt Easton I was convinced that my sluggish pace was due to the recurrence of a previous mechanical.
The frequent stops to investigate that (and finding nothing wrong) + the headwind and my inability to generate any power at all meant shedding every bit of time buffer built up northbound, despite riding through the night and the efforts of my riding partner to tow me along.
Finally about half way from Gt Easton to Loughton I diagnosed the problem: the 'drag' was my right leg preventing my left from generating full power. Once I knew to let the right relax and 'freewheel' and focus my remaining energies into the left I was able to finish with 20 minutes to spare.