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If you really want as car free as possible I'd look to combine something like the paths alongside the River Lea to Cheshunt ( http://www.sustrans.org.uk/sites/default/files/documents/london20docklands20leaflet.pdf ) and then the bottom bit of this: https://www.strava.com/activities/162788353 which goes that appropriate way on relatively quiet roads.
If you want roads and it's early then I tend to use the A10/A1010/B176 to get to Cheshunt: https://www.strava.com/activities/163173147 but it's hardly car free and rather functional (i.e. get out of London relatively quickly).
Never done the River Le[ea] path but some on here have.
Does anyone have a good (read nice) London - Baldock or Royston cycle route?
On road, and as car free as possible. I'm mostly finding things that look horrid :(