Check out the Meetups forums <hong-kong.meetup.com> for running/hiking/marathon training etc etc.
HK is extremely good for trail running - I found training for a road marathon a little more challenging as there were limited options for flat mileage training. However I run throughout the year and my slow summer slogs pay-off when humidity and temperature drop off in September as you find your speed and stamina massively improve.
Check out Action Asia Events and also Seyonasia.com