You can call the Hong Kong Tourism Board visitor hotline for enquiry: 25081234 (press 4 then for Cantonese speaking operator). The operator will help you finding the route to go to 南生圍.
香港旅遊協會 is the former name la. Now it's called 香港旅遊發展局. Sorry ah, Miss. I am not very familiar with Sai Kung, espeically the walking trails. I have found the government site: http://parks.afcd.gov.hk/newparks/chi/hiking/index.htm for you. You can check if any route is suitable for you in Sai Kung. If ask for suggestion, call the number that I mentioned in the previous msg.