Check out this web site

http://www.zuerich-athletics.ch/


Also there is a Light athletics track and field open stadium

Sportanlage Sihlhölzli
Manessestrasse 1, 8003 Zürich ZH

I don't know if you need to register and pay something to use the track, but I saw people just coming and using them.

If you live in Dübendorf, there is also one at

Leichtathletikanlage Eichstock
Eichstockweg, 8600 Dübendorf ZH