Walking Challenges

Does anyone know of a site that does walking challenges - when I did slimming world you could go onto their fitness page if you were a member and pick an activity say - walk round Central Park, walk along the Great Wall of China etc and everytime you did some exercise you logged how many miles you did and eventually you would complete your challenge. I loved it and it kept me really motivated but I can't find a similar site which is free for everyone.

I know I could log it myself but it is so much more fun seeing your little person move along bit by bit.

Thanks

Gillian