Greenwich foot tunnel is pretty cool, and everybody just rides through it even tho it says not to..
If I were you id head back along Regents Canal, great ride.
From Greenwich take the foot tunnel, isle of dogs, Thames path round to Limehouse Basin where you pick up the canal then follow the path all the way back round to Regents Park. it goes in a nice arc through Victoria Park which is really nice, Haggerston, South Hackney, Islington (slight road detour through Angel) back on through Camden and Primrose Hill then your onto the path into the park through the London Zoo Avery. From there you can filter down through Hyde Park, go past the palace, Westminster n’that, and back onto the Thames path.
you could even detour along the Limehouse cut and follow the canal through Olympic village if you wanted even more mileage!
loads of pubs and cafes along the path too.