Depends on whether you have an aversion to using roads or not.
Not sure where you're starting from in Mitcham but try this on-road option.
Mitcham Road to Canterbury Rd, turn right.
Left at Terrar Rd
Claim the lane through the roundabout turning right into Hanover Rd
Cross Burwood Hwy to Harland Rd (as straight as you can go at the lights)
Left at Weeden Drive, right into Highmont Drive
Right at Highbury Road all the way to Warrigal Road
At Warrigal Road, go straight ahead onto the footpath and turn left then right beside the tennis courts (walking your bike on the path of course)
Turn left into Queens Pde at the bottom of the carpark then right into Bath Rd
Left then right to cross the railway line then left on Prosper Pde.
Follow around the corner to Yuille St then left into Summerhill Rd.
Cross High St (turn right then immediately left) into Hilltop Ave
Right onto Dent St all the way over the Freeway to turn right into Malvern Rd
Left onto Wattletree Rd
Left at Kooyong Rd, cross Dandenong Rd, right into Alma Rd (sticking to Wattletree to Dandenong is more direct but far more traffic and traffic jams)
Right at Chapel
I used the above route between Highbury and Mitcham commuting to Cadbury at Ringwood and the other part commuting to the St Kilda Road office. There are some decent hills along Highbury Road and up and around Hanover Road but nothing seriously big. Overall distance (depending on the start) is about 25-30km. Traffic volumes are pretty good and there is a number of on-road bike lanes to give you a clear run through the congestion (esp Alma Rd).
Hanover Road has a bike lane. The motor traffic exiting the Coles supermarket car park regularly ignores the law along there and you must watch for the pushy ones.
Terrara Road/Hanover Road roundabout has green bike lanes. Avoid them like the plague when you turn right as they allow the overtaking motor traffic priority and invite them to run you down as you try to turn right: get in the main lane, signal your intention, take your time and keep control. Be aware too, that the camber of that roundabout is all wrong and take it slowly.
Major intersections along Highbury Road have added right turn lanes. This makes the other lanes dog-leg left a bit and makes them a bit narrower. Motorists tend to take the straight line which means they'll shave you on the exit if you don't claim the lane here too. Sit right in the middle and don't be bullied.
All the cautions above should not be interpreted as meaning it's a dangerous route at all. I found it very safe and relatively lightly trafficked but, if there was going to be a scary point, those were the places it would happen.
If you want to start off with a lesser distance, you could drive to Queens Pde off Bath Road and park behind the tennis courts initially.