Dragon's Back is the only MTB trail in Hong Kong Island. About 10 minutes drive from the nearest MTR station, 20 minutes uphill climb on a bike. It's about 6km offroad, short but very close to the city.
It is a popular hiking trail so try to go on Saturday instead of Sunday. Go early too, say before 9am.
I go there during weekends. Let me know if anyone need direction or a guide.