Following email asking about easier hikes, here are some more ideas for this thread – sometimes way shorter than 20km:
Circuit of the Peak on Hong Kong Island is perhaps our best nearly level "hike".
Strolling along Cheung Sha beach, south Lantau Island, is even easier. Could be combined with walking elsewhere on Lantau, such as to the main Silvermine waterfall at Mui Wo.
Walk along Peak Road on Cheung Chau is relatively easy; this is more a wide footpath than a road, and undulates along southern "spine" of the island.