Length: 12 km (7.5 miles); see † below for a longer walk. Toughness: 1/10
10:28 Cambridge North train from Liverpool Street (Tottenham Hale 10:41), arriving Cheshunt (in TfL Zone 8) at 10:48. You can use Oyster PAYG or contactless, but note that a Freedom Pass is only valid to Cheshunt on the half-hourly London Overground services via Seven Sisters (arriving at xx:26 & xx:56).
There are fast trains back to Liverpool Street at xx:20 & xx:51 and slower ones to Stratford at xx:29 & xx:59, plus the Overgrounds at xx:00 & xx:30.
This walk combines the extensive wetland landscape of River Lee Country Park with a short excursion into the town of Waltham Abbey for a circuit of the Abbey Gardens and a peek into its fine Norman church. The town's pubs didn't all survive the pandemic so there's currently no clear recommendation for lunch, but the walk document suggests trying the Welsh Harp in Market Square, next to the church grounds.
On the return route it's worth popping into the Brew Street Kitchen and Bar in the Lee Valley White Water Centre, either for some mid-afternoon refreshment or simply to watch the spectacle. Much of the walk is along surfaced paths, but be prepared for a few muddy stretches along the way.
You'll need to bring the directions from the L=swc.311.a page.
† For a longer walk of 17½ km (10.9 miles) you could do the full Main Walk, ie. continue past Cheshunt to return from the next station on the line. Broxbourne has four trains an hour back (not the Overgrounds), departing four minutes before the times at Cheshunt. See the walk document for more details.