Sam Anderson posted the route:
The Lee Valley Way is a 53 mile route along the Lee (or Lea) River from its source in Leagrave, Luton to Leamouth at East India Dock, where it runs into the River Thames.
Use this link for maps: tfl.gov.uk/modes/walking/lea-valley
The signage is all pretty clear apart from between Welwyn and Hertford. It is a good idea to take the maps though or I can give you my gps route to download onto a watch.
Here is my data from strava: www.strava.com/activities/661281045