Length: 15¾ km (9.8 miles). Toughness: 3/10
10:28 Welwyn Garden City train from Moorgate (Highbury & Islington 10:36, Finsbury Park 10:42, etc), arriving Alexandra Palace (TfL Zone 3) at 10:49. Use Oyster PAYG or contactless.
Anyone wanting a later start and/or a shorter walk could join at Highgate tube station (Northern line). Be there by 11:45 to intercept the main group, who should be passing the Priory Gardens exit about then.
At the end of the walk there are frequent Greater Anglia trains from Tottenham Hale to both Liverpool Street and Stratford, plus the Victoria line. If you want to finish early there are many places where you can hop on a bus or head for a nearby tube station.
This walk is essentially two short walks joined together. The morning walk is mostly along the well-known Parkland Walk (a disused railway line) and the afternoon walk links two large nature reserves, Woodberry Wetlands and Walthamstow Wetlands (but note that these both close at 4pm).
For lunch I suggest one of the cafés along the route, either in Finsbury Park or at Woodberry Wetlands. There are other cafés towards the end in Springfield Park and (if you get there in time) Walthamstow Wetlands, plus a pub opposite the reserve exit if you need something stronger.
You'll need to bring the directions from the L=swc.321