The main one is the Grand Union Canal. you can access the towpath at Camden Lock (just north of Camden Town), near Angel Islington, near Lords Cricket Ground etc. It eventually runs to Birmingham via Uxbridge,

Watford, Tring, Milton Keynes. Another arm of it runs from Southall in West London into the Thames near Brentford

For a really picturesque walk take the Underground Metropolitan Line to its terminus at Uxbridge and walk up the canal to Rickmansworth, also on the Metropolitan Line. It runs through open countryside and nature reserves on this section

Grand Union Canal; Regents Canal; Lee navigation

All the navigable canals in London have towpaths, except through the two tunnels on the Regents Canal.

Regents Canal, north London

There are several canals with towpaths in London.

The Canal and River Trust have a list.

Yes, what about them?