Length: About 12 km (7.5 miles). Toughness: 1/10
Take a Central Line tube (towards West Ruislip) to arrive at Greenford by 10:45. These run every 5-6 minutes and take about 30 minutes from central London. Meet up in the station foyer, not the platform.
Return is by Piccadilly Line tube from Osterley (every 5 minutes). Greenford and Osterley are both in TfL Zone 4.
This west London walk goes through lots of pleasant green spaces and you can almost imagine yourself in the countryside when you reach Osterley Park, a grand estate managed by the National Trust. There's no charge to enter the parkland but non-members would have to pay an entry fee to explore the formal Gardens or visit a ‘Winter Exhibition’ in the basement (the main house is closed for winter). Note that the park closes at 4:30pm.
The walk notes still recommend The Plough in Norwood Green as the lunch stop, but this comes quite near the end of the walk. I see that a couple of people have suggested an alternative hostelry just off the route, The Fox in Hanwell; it's now mentioned in the directions and might be worth a look en passant. The tea stop is the NT café in Osterley Park.
You'll need to bring the directions from the L=swc.42 – they've recently been revised to take in a small canalside park between the two pubs.