Good suggestions already.
As Tassie is quite mountainous, going up hills and mountains is good training for building your fitness for multi-day walks, as you will always encounter a hill climb on a multi day walk (except Hazards beach to Cooks beach, perhaps).
Taking a pack and slowly increasing the load is also a good training thing so I have read.
This topic shows a walk that is probably not a good one to start with unless you already have pretty good fitness, better to go up Mt Wellington a few times to get your fitness right for it.
You can read about my first day walk in Tassie there...