Length: 19.6 km (12.2 miles). Toughness: 3/10
10:12 Peterborough service from Kings Cross (Finsbury Park 10:18), arriving Sandy at 10:58.
Trains back from Biggleswade to Kings X are hourly at xx:19. Buy a return to Sandy.
I haven't done this Bedfordshire walk myself but it's pleasingly described as a mix of grassy tracks and woodland trails; “no slogging through crops or over ploughed land”. There's just one gentle climb halfway round, so although it's not a short walk† it doesn't look at all strenuous.
The walk passes through “two lovely villages, both with excellent pubs”: Northill after 5 km and Old Warden after 12 km. That means they're not ideally placed for a regulation lunch stop but you could opt for a two-stop strategy; perhaps a starter in one and dessert in the other, each washed down with something reviving. Biggleswade has a plentiful supply of watering-holes but it's becoming quite rare to find a café that's still open at the end of a Sunday walk, so you'll probably have to hang out in one of its pubs if you've got a long wait for the hourly train.
† Before anyone asks the walk document doesn't list any short cuts, but if you had a map you could try to follow the North Bedfordshire Heritage Trail south from Northill via Ickwell, intersecting the main route near Shuttleworth Aerodrome; that would reduce the walk to 12.2 km (7.6 miles).
You'll need to bring the directions from the L=swc.264