Length: 14.2 km (8.8 miles). Toughness: 2/10
10:45 TfL Reading train from Paddington (Ealing Broadway 10:53), arriving Maidenhead at 11:25. Buy a return to Marlow, unless you've got a Freedom Pass and don't mind a longer wait at Maidenhead for a TfL service on the way back, in which case you'll only need a single for the return leg from Marlow to Maidenhead.
Return trains from Marlow are hourly at xx:55, changing at Maidenhead.
This is an easy walk along one of the prettiest stretches of the Thames, where the wooded escarpment below Cliveden is said to be “especially lovely when the leaves are turning in autumn”. If you want a pub lunch there are at least three establishments† in Cookham to choose from, all summarised in the walk document.
The afternoon leg includes a stretch along a wooded ridge with fine views of the Thames. Marlow's independent tea places might have boarded up their shop fronts by the time you arrive, but the town isn't short of refreshment places along its High Street.
You'll need to bring the directions from the L=swc.56 page.
† Odd pub name: “Bel and the Dragon” is a biblical reference to two episodes added to the Book of Daniel, although neither tale provides a convincing explanation for the name of a gastropub.