How are you getting back? Topanga's not terrible getting there, but coming back sucks donkey *ick. Coming back out of Malibu is pretty much shoulder-less and a hurty climb. If you have a better route back (Latigo or something), it won't be bad.
Depending on where you start, it's less than 15 miles one way.
__________________ If ignorance is bliss, shouldn't you be happy?
of all the possible routes through the Santa Monica Mts, I never understand why people want to ride on Topanga. It's the busiest single lane road in the US according to some estimates. Add to that moms in SUVs on cell phones with kids screaming in the back and it can get pretty ugly. A better choice would be to meet in Topanga (Pine tree circle), ride up Fernwood, down stunt, over to cold canyon and up Piuma back around to Fernwood. A much better ride - better climbing, much less traffic, better scenery.
Mulholland Hwy is more or less parallel to PCH. There are several canyons connecting them, and Mulholland itself ends at PCH. If I'm going on Topanga, I like to climb Old Topanga on the valley side. I also don't like to climb Topanga from the beach, but it can be ok if the traffic isn't too bad, but the bottom is steep.