Within Hillingdon there are 3 long walks that I know of: London Loop, Hillingdon Trail and Celadine walk.
I have done the London Loop and Celandine route while the Hillingdon Trail is on my to do list.
Ruislip Lido and the woods around it (Park, Bayhurst, Copse and Mad Bess Woods) provide good walking too.
Cycle wise I tend to stick to the Grand Union Canal. To the north I've been as far as Hemel Hempstead, to the South I've followed it to the Thames at Brentford and then used the Thames Path on the south side to get to Kingston and then the north side on the Thames to Hampton Court. Eastwards the canal takes you all the way to little Venice near Paddington and if you take the regents canal from there you can get all the way to the Docklands though you need to navigate some roads near Lords and again at Angel where the canal enters tunnels.
I have a walks page that may be of interest if you are looking for walks around london
http://ericbwongderivatives.blogspot.co.uk/p/walks.html