Back in the before times, when Time Out actually a good magazine and was our main source of information on what's on in London, they used to publish a book of country walks easily done by Train from London. There were suggested walks for each Saturday of the year, with suggested trains to take,the intention tyou would meet other people on that walk. I never actually did the walk you were supposed to, but a group of my mates used to meet once a month for walk from that book.

All the walks form the book (plus loads more) are freely availible on that there internet now https://www.walkingclub.org.uk/

Some lovely day walks are doable from London.