This walk is recommended for May when azaleas are in bloom, but I've found it to work well in the winter. There are good surfaces and some welcoming hostelries in Windsor to cheer you up in the gloaming. You can always go back in the spring.
Travel: 1020 from Waterloo (Clapham Junction 1028) arrives Sunningdale 1108. Return - from Windsor and Eton Riverside, 2 quite slow trains an hour to Waterloo at 23 and 53. If returning this way you'll need a single from Windsor to Staines as well as a return to Sunningdale OR a return to Staines and a single Staines-Sunningdale is cheaper if more of a faff.
From Windsor and Eton Central, there are trains at 20 and 40 to Slough where you change for Paddington. This is a bit quicker even with the change. You'll need two single tickets for this option.
Lunch: The only lunchtime stop is the Savill Garden Kitchen, in the Savill Building, about 5 miles into the walk (just as you reach Windsor Great Park). Unfortunately this seems to have become rather pretentious and pricey. You can't book in advance, apparently.
Tea: There are plenty of pubs and tea shops in Windsor to choose from for a post-walk tea stop. The main walk page recommends The Boatman (tel 01753 620 010), a gastropub and riverside restaurant located directly behind Windsor &Eton Riverside railway station or The Cinnamon Cafe (01753 857 879) in the Windsor Royal Station retail precinct (site of former railway station terminus).
Shortening the walk: once in Windsor Great Park you don't have to follow the exact route and could head more directly to the Long Walk and the park exit, especially if it is getting dark.
*The park closes at 5*
For walk directions, map and GPX click here
T=1.18