So apart from the Nepean Highway cutting a swathe across the middle, it's dead straight forward. So this morning I decided to run from McKinnon to the beach (due West), along the bay a bit (I can't call it a beach being from Manly) and then back home. All went well, running parallel to North Rd and reaching the bay in perfect early morning weather. Then I ran back, heading due East. Except, after an hour, I find myself in a suburb I didn't recognise (Hampton - which was very nice). So I panicked a bit and ran more directly home, except I then end up in Sandringham. Which is by the Bay even further away. So having been out for 1 hour and 15 and miles from home with no idea how to get back, I did a Rob Costello and caught a taxi.
Take a look at the map again. It's all NSWE grid lines apart from a random section in Brighton which looks like a fault line or earthquake has hit it. That was my problem.
15km, probably.