Anne, I share your feelings about not being able to give my boys off-leash time. We have a fenced-in backyard, but it is full of trees and we have hawks. Our neighbor's Yorkie was attacked by one.
Like you, I give my boys plenty of walks daily and we play indoors, so they do get lots of exercise. We did find a baseball diamond, fully enclosed, that we can let the boys run free on occasion, but I don't feel 100% comfortable doing that.