This is a fantastic article on getting fit in the great outdoors courtesy of Go Outdoors. I think there's also other benefits in choosing to train outdoors - you can get all your equipment together which will no doubt be super handy when it comes to surviving the zombie apocalypse.
If you can't use your home as a fortress, you're going to have to escape and get somewhere a little more remote. Having broken-in hiking boots, all the camping and survival gear, and knowledge of the surrounding terrain will put you in lead when it comes to surviving the undead. Thanks Go Outdoors!