Length: 17km (10m)
Toughness: 3 / 10
Transport: Take the 10:15 train from London Charing Cross, change at Tonbridge for the 11:04 to Yalding arriving at 11:18. As Sean pointed out in the comments below you have 6 minutes to change platforms in Tonbridge with this connection. If this is too tight, take the 10:10 from London Charing Cross and change at Paddock Wood which requires a 13 minutes wait on the same platform. Return trains from Borough Green are at xx:18 and xx:48.
This is a pleasant walk in Kentish countryside, passing the Elizabethan Manor house Roydon Hall before breaking for lunch at the cozy Swan on the Green at West Peckham (follow Option 2 of the instructions, they usually have still cider for those partial to that beverage). The walk continues through Mereworth Woods before finishing in Borough Green. There is a newish cafe called Melias Place (open until 17:00) which looked attractive from the outside last time I passed by.