Check out the London walks website: www.walks.com to see if anything appeals.
I followed the Line Walk last November. From the O2 to the Anchor and Hope pub. Saw nary a soul.
Richmond Park: for a long stroll take a train to North Sheen station,( use your Oyster Card ) Exit and follow Manor Road a few blocks to Sheen Road and turn Left. A few more blocks and enter into Richmond Park on the Kings Road path directly adjacent to the East Sheen Cemetary. Lovely stroll in a beautiful park and if you are lucky you’ll get to see the red and fallow deer. You could walk all the way to Isabella Plantation or to the Ham House which is on the Thames River Path. Richmond Park is great.
Or tube to Hampstead and follow the Flask and then Well walks into the Heath. Seek out the Jim Henson Bench and a pleasant POV of London.
Or see if any of the Jubilee Walks suit your interests
Or tube to Paddington Station exit and follow the river path to Little Venice.
Just a few walk suggestions. Over the years have done all of them. Enjoyed each.
EDIT: Near the O2 is the architecturally interesting Design Center. Its where I started the Line Walk.