Londonist web site's advice and walked from Erith to Greenwich along the Thames path, and jolly good it was too.
According to Google Earth (above) it's around 13 miles - just over if you count the walk to and from the train stations (plus a bit of pootling about down jetties and diversions for ice creams).
The smelly bit mentioned in the earlier post is only a short distance but boy is it pungent: hold your nose and breath through your mouth for a bit.
Keep a look out for the Grandad sign on one derelict jetty while you go:
Arsenal (go the Gunners!) and this sculpture:
The biggest sights (in every sense) were yet to come....