Ah, I read your post too quickly. I'm not as familiar with getting out of the city at the west end, but I don't think there's a terribly easy way (west enders, correct me if I'm wrong -- I just don't know). I'd still recommend taking the GO. Even if you have to transfer to a bus, it's not bad at all. All the buses are equipped with bike racks at the front and it doesn't cost anything extra to bring a bike. You'll at least bypass a decent chunk of urban sprawl, which sounds like something you might like to do, based on the fact you're checking out nature reserves.

Whatever you end up doing, I'm interesting in hearing how your trip goes!