Thanks for all the advice. Just got back from 9 days walking in the Zillertal. Loads of good walking to be had all round the main valley and the side valleys. Best tips are:
Get and ActivCard - gives unlimited travel on buses on trains in the valley and one trip per day per lift to gain height rapidly to start walks.
Main mountain areas with lift access are Penken, Eggalm, Gerlosstein, Ahorn and Hintertux.
Several good valley walks too, the Stilluptal is a must for the scenery (minibus service to valley head), The Schlegeiss is also a must - accessible by bus, toll for last section up to lake. Zillergrund valley also looks good but we did not have time to explore it.
We ended up doing more valleys than hills because the weather was predominantly wet and murky with the snow line down to below the tops of the lifts for a couple of days. Definitely going back as I have a raft of planned walks left undone as a result.