Well, we made it. It was a proper adventure. There was much more non-tarmac riding than I expected, ranging from gravel paths to rocky trails and single-track. Our decision to travel light-ish and stick to one pannier each was a good one in that the bikes and wheels held up to the mistreatment.
The final ride into Istanbul itself, including a scramble down to and crossing over an ancient roman or ottoman aqueduct was pretty mad.
I learnt (again) that the people of this world are overwhelmingly kind and welcoming.
We raised over ten grand for research into children's blood cancers.
There's a daily report and pictures
here https://www.facebook.com/GeorgeandJohnsRidetoIstanbul/ if you can bear to click on Facebook. No login should be required.