Mt Victoria is a great place to ride close to the city. You can be in the midst of the action within 5 minutes of the CBD. Mt Vic offers all year round riding options that are great for a lunchtime ride or for a quick night ride after work.
Mt Vic is flush with wide walking paths (easy) and rooty single track (easy to intermediate). To connect to both sides you're likely to have to ride up, or down, Alexandra Rd at some stage.
The paths are all dual use so they are likely to be used by walkers, runners and dog owners. Please ride a little conservatively if you are out there at peak times.
Most of the good cross country rides run between the lookout at Oriental Bay and the Newtown end of the reserve (parallel to the main walkways). Once you've explored a bit, you'll easily put together a fun 6-8 km loop with lots of alternatives.
There are some decent downhill options running from the Hataitai Park side towards town. Many of these runs cut across the main walkways so down-hillers please take care at the points where runs cross main walkways. Conflict between riders and walkers could cause the council to rethink their access policy.