lol Wayne, stick to swimming, less wear on your joints ! Take my hat off to runners, but don't wish for the pain ... except for the euphoric high when the endorphins kick in. I'll stick to walking ... unless being chased.
Read a funny book called "Fat, Forty and Fired", for some one training for an event, not directly applicable to this thread, as you run already.
Anyhow, wish you the best with the Sydney Half Marathon, while I am not unfit, and still am relatively young, personally, for my joints - I feel pain just writing or thinking about this - that is jogging on hard road.
Anyhow go for it and best of luck!