Tucked into a residential neighborhood in Camarillo is a relatively small, very low key park called Arneill Ranch Park. This is an older park surrounding by homes at 1301 Sweetwater Avenue, just west of Arneill, between Dunnigan and Rocklyn.
While I would not go out of my way to visit this park with my kids (there's a small playground and swings for the local neighborhood kids), what is particularly unique about this park is that there is a full 1/4 mile hard-packed dirt track that circles the park. Nothing overly fancy, but a great opportunity to do some track/interval training or just jogging/walking in a controlled, measurable fashion. It is quite rare to find a park with a running track like this.
The park also has some decent, fairly newish looking fitness stations around the track, as well as restrooms (these are not newish looking).
More info at www.pvrpd.org/parks/active/arneil.asp.