Originally Posted by
DaveReidUK
Another option would be to jump on a Tube from T4 to Boston Manor, which is a stone's throw from the canal at Osterley Lock.
Likewise you could take the Heathrow Connect train from T4 to its first stop outside the airport, which is Hayes, next to the main line of the canal, then walk east along the towpath to Boston Manor underground station, probably 1.5-2 hours walk, then back by Tube.
Longer walk, go the other way from Hayes, westwards, nicer scenery including some quite rural stretches for London, some canalside pubs, to Uxbridge, then back on the Piccadilly Underground changing at Acton Town.