We’ve always ranked Hong Kong as the city with the best hiking within easy range of local transit.
It’s included in one list posted by Matador:
- Crosstown Trail — San Francisco, California
- Arthur’s Seat — Edinburgh, Scotland
- Lakefront Trail — Chicago, Illinois
- Elliot Bay Trail — Seattle, Washington
- Victoria Peak and Pok Fu Lam Country Park — Hong Kong
- The High Line — New York, New York
- Mount Royal — Montreal, Canada
- Schuylkill River Trail — Philadelphia, Pennsylvania
- Indianapolis Cultural Trail — Indianapolis, Indiana
One hike that’s easily as good as those is in Valencia, Spain.
besthike.com 
How about the 78-mile long, 15 stage, Capital Ring in London? Taking you over the river (twice), through the 2012 Olympic Park and past former royal palaces and royal hunting parks, and taking in some of the city’s finest views. And all designed to be reached by public transport. Is there another urban trail quite like it?
https://tfl.gov.uk/modes/walking/capital-ring
Looks great. I’ve been in London a few times recently myself. This would be right up my alley !