Off the top of my head, I think it's around 22km's. As far as navigating it blind for the first time it can be done, but you better make darn sure you have really good maps and you have the guys at Hartley Bay go over in detail exactly how to get down.
In the past I've seen guys with big console boats navigate up and down those narrow chutes, but you're much better off with a tiller...Much more direct and instant response when sharp turns are required.
Would also mention that going back upstream, slowly and under control is MUCH easier than hurltling downstream....
If you can make it down to the Cross Channel you're golden....
Bear in mind as mentioned you don't have to go down there to catch lots of fish and have a great trip...It's pretty nice upstream as well.
BTW, if you do make it down to the delta of the Bad and camp there, there's a really interesting phenomenon that occurs down there in late June to early July. Every evening at dusk millions of shad move in from Georgian Bay (I guess to spawn?), and the entire river comes alive with fish of every species going on a frenzy. It's really cool...