I do something sort of like that, but not exactly. I know where I want to start and finish, and some of the major waypoints in between. But much of the story writes itself in between those waypoints: characters do things, the world reacts, you explore the consequences, and vice verca. I've come up with some pretty weird inter-story tangents before because sometimes life just happens, even in a story. As for defeating writer's block, I usually don't suffer from it too much because I simply walk away from stuff if I'm blocked and come back when I'm not. Until I actually try to sell something (and Black Tiger might well be the first) then it's a hobby, nothing more. The hardest thing for me is simply starting a new story- that first blank page. The way I defeat it is to just start writing. More often than not, the first chapter ends up being one of the most heavily re-written, but that's okay. By that time I'm off to the races. BT is at about 220 pages right now and I figure I'm about halfway through the story. I've been hitting it pretty hard.
^^^ That's mostly how it works for me, too. I have a key scene or two in mind (which may or may not be the beginning or end) and I have to figure out how to arrive at that scene. The characters, once fleshed out, do dictate their own actions to some extent.
I was writing a scene today and had another episode of the story just going its own way despite what I planned. Long story short: meeting between survivor of the big battle and the parents of his shipmate who got killed. Original planned scene was going to be sort of a teary group hug between the guy and the aging parents- it turns out the dead guys' parents are a couple of younger liberal-minded 'hipsters' who hate the Navy and are pissed as hell that their son died in the Gulf. Not how I planned it at all, but when I started writing it it just came out that way. Some days even I just have to say: What. The. Fuck?