So, this is a proposed two-stage walk. It hasn’t yet been done.
The first stage is the Parkland Walk, following the disused railway lines from Ally Pally to Finsbury Park. This is ~8km and expected to take approx. two hours.
The second stage, still in the planning process, will take a route south from Finsbury Park via Highbury, Islington, Clerkenwell, The City and St Pauls, ending on the south side of the Millenium bridge.