Just considering the ECE options for this from East London, looks like 62km from the Isle of Dogs directly to Witham, and should be OK with no intermediate control (McDs between Shenfield and Mountnessing for emergency coffee/loo stop). Return to Upminster station makes it 103km on Google Maps following the outbound route most of the way. BUT don't think C2C are going to be tandem friendly so another option is head westwards to finish at Sawbridgeworth or Harlow which do have a train service that day, potentially nicer roads too. Comes in around 106km.
Anyone for meeting up for an East London ride out? Our route is very unexciting and involves Ilford, Romford and Brentwood, so more inspired suggestions welcome!