There are several great walks in this part of the parklands. If you start at twenty seventh avenue and follow the paved trail to the pedestrian bridge you can take a left and follow the dirt road till it loops back to your starting point. Rougly 4km and a pretty easy one hour walk. If you start at the pedestrian bridge and take the road as far as the dam before following the tracks behind the dam out to the offtake reservoir and returning via the powerlines track its about an 8km walk and will take roughly 2 hours.
You could start this walk at several places but my usual start point is parking on Dobroyd Avenue in Elizabeth hills and using the flyover bridge to cross the M7 into the parklands.
Another Good starting point be the parklands track entrance where twenty seventh avenue and mciver meet.