Cottesloe huh!?
if you dont get sidelined by the bikinis then there is a group ride that heads off from fremantle every saturday and sunday (and public holidays) at 630 in the morning. Its called the old papas ride and it leavesfrom ginos cafe (on themain cafe drag in freo). Freo is only about 3 kms from cottesloe. Not many good roads for riding along the beach in perth though there are heaps that go around the river and the majority of group rides concentrate on going around the river. Around the river from cottesloe will give you 50kms the short way and around 70-80 the long way.
Alternatively you can go "hills" which will give you around 100 clicks for the day from cottesloe. On the wa cycling website they have a list of all the rides on in WA. Go to
www.wa.cycling.org.au and go the events tab. From there select the "group rides". Best rides are old papas, barrack st, stockbrokers and glen parkers, from my knowledge at least. I'll see you on papas!