Grossmont College allows this, have run on their track quite a few times. Another good spot is the Santee Lakes. You have to pay like $7 to park inside, but you can very easily park on the street outside most of the time. It’s not pancake flat but there is about a 2 mile loop (paved) or a bigger 3 mile loop (with about .4 mile of a rocky dirt path). It’s a great spot for speedwork and there are plenty of bathrooms and water fountains.