there is the Otay Lakes and Olympic training center toward south. You can push past that and go to campground I think it is thousand trails. There is a pretty challenging climb past the lakes called honey springs would definitely push you over the 30 mark.
You can go west toward the coast and downtown on a bike path.
I don't know the area perfectly so as far as miles I think that either route should be close to 20-30. Each time I have ridden the area I was in a group ride and just mindlessly following. This should give you a good start but I would check with a LBS for better info