As
@srw said, this was my first long ride on a solo bike in a very long time, and riding the tandem is different in a number of respects! The back problem was the thing which really did for me in the last few miles, but I thoroughly enjoyed the night overall. I'm just sorry I wasn't able to do any riding on the French side, but at least my presence in the team car meant that
@srw could get a few miles in.
Resolutions for the future: (a) I will be back! once I've re-trained my back to cope and (b) I will learn how to do hills properly. Thanks to everyone for keeping me going and waiting around when necessary!