Saikung promenade : lots of dogs to play with followed by Seafood
Both sides of Tolo Harbour, The banks of Shing Mun river, MaOnShan : All nice&easy, flat&paved
The walkway between TaiPo waterfront park and TaiMeiTuk
Some above are fairly short (Saikung) while others are quite long (TaiPo) but can be escaped by flagging down taxis.
If you want something away from people, then look into Tsim Bei Tsui