Salsa_Lover has mentioned
Eichstock already, but I'm not sure if it is public of reserved for club members (see link). Then there's also the
Dürrbach complex with six tracks, same applies here.
Here's a list of sport facilities of the city of Zurich (without the baths). They're usually public but club members may have priority on some of them. The mentioned Sihlhölzli is the biggest facility to my knowledge, but it is situated near a busy road. I'd check your options in your own commune first, it's an infrastructure many of them can provide.