The Lea Valley walk is a lovely stroll for Londoners at any time of the year, but in autumn I think the combination of trees, leaves and water is particularly lovely.
For an 11ish mile walk, I recommend heading north from Limehouse to finish up at Ponders End, where you can catch the train back to Liverpool Street Station. For a shorter stretch start or stop at Markfield Park, about 1/2 mile from Seven Sisters and Tottenham Hale tube/train stations.
En route you pass the surprising and stunning Three Mills, the Olympic Park (albeit through security fencing), and breathe in the fresh air of the great green expanses of Hackney, Walthamstow and Tottenham Marshes - and then you're in the countryside with narrowboats and fishermen.
The whole route follows the reflective waters of the Lee/Lea in its various guises - from the Limehouse Cut to the Lee Navigation to the River Lea.
My favourite place to break the walk for a bite to eat is Pistachio's in the Park Cafe in Markfield Park which runs alongside the Lee Navigation.
For a coffee early on in the route, and good food too, you can detour to the Counter Cafe in Hackney Wick - it's on the west side of the canal at the junction with the Hertford Union Canal - which itself offers up a whole host of alternative destinations!
For me, the hardest thing about doing any of the Lea Valley Walk was working out how to get onto it. If you are walking from Limehouse aim for St Anne's Church - it marks the point where Commercial Road crosses Limehouse Cut, and there's access to the canal there. St Anne's is also one of Hawesmoor's churches and its website is great for directions - you can get there easily on the No 15 bus from central London, or on the DLR (Limehouse/West Ferry stations).
Lea Valley Walk information: www.walklondon.org.uk/route.asp?R=4
A Walk Along The Limehouse Cut Canal (lots of info about what you'll see on the early section): www.imvisitinglondon.com/limehousecut.html
Three Mills: en.wikipedia.org/wiki/Three_Mills
Pistacios in the Park: pistachiosinthepark.org.uk/category/markfieldpark
Friends of Markfield Park (good map): www.markfieldpark.org.uk/
Counter Cafe: thecountercafe.co.uk/
St Anne's Church website (great directions): stanneslimehouse.org/location.html
TfL bus route map finder: www.tfl.gov.uk/tfl/gettingaround/maps/buses/