I don't know the mileage, but I enjoyed a wandering run around and through Kensington Gardens and Hyde Park. You can run the perimeter and then do a lot of cross-park running. Very beautiful, generally surfaces that are easy on the legs (I try not to do pavement any more).

Good luck with the Phoenix Marathon. I did the Lost Dutchman a couple years ago. Great time to be running, although still a bit warmer than I expected...