Ah you are having the same problem I had when I used to run games, You think of the goal and the great climatic battle at the end but you don't have anything in-between. Keep the ending of course, but put it aside for now, after all it doesn't sound like it is going to happen for a long time. You need to work on short one-day adventures that will slowly foreshadow what is to come, or they can have nothing to do with the main plot; I find a mix of the two works best. Each of these sub plots shouldn't take more than one or two sessions to finish each, just in case something comes up and the game needs to be put on hold. This way you will have a good place to leave off and easily come back to once you are ready
Now, as for the sub plots; they can very depending on the group and of course the theme of the game but there are four main themes that work in any setting: Action, Drama, Romance, Horror, and Comedy. These can be mixed together but I don’t have to tell you that.
From my experience what works best is if you think of your game more like a TV series than a big full scale movie in the sense that each session will have an opening and a climax of there own. Perhaps each one having it's own "boss" or whatever. For instance, you could have them stop in a typical eastern European style village that is in constant fear of the unknown menace that has been snatching villagers who venture too close to the woods. Now you could do several things with this you could make it a horror and have perhaps some sort of intelligent undead or maybe a werewolf snatching people. You could make it more specific by having it be children that are being taken.
Perhaps you could add a Comedy twist by saying that the Werewolf isn't killing the children; just taking them to his cave where he cares for them. The children have come to love him, perhaps the first time one of your muscle-brained players tries to attack the "dangerous" creature the children surround the werewolf, or the PC as they hit the big mean man. Maybe only a few people has disappeared, maybe the local lords daughter, perhaps she was in love with the werewolf before he changed; but the lord forbid it because the soon to be werewolf was a commoner. The commoner, pretty damn upset, went off into the woods to get firewood and take his mind off of his problems and is attacked and bitten by a werewolf. Now he is living in the woods unable to bare being in public as the beast he has become, he is the first to disappear. The lord’s daughter, distraught, runs to the woods to find him and be with him despite her father and isn't seen again. The lord has the players go out into the woods to find her because some large beast has killed his own men whenever they enter the forest. The players find the daughter and the commoner, after some talking and they have decided on a course of action the other werewolf comes bounding in. There is a large battle at which point the PC’s and the commoner/werewolf have to kill the other werewolf. Maybe this changes him back, perhaps he dies in the battle to save the lords daughter, it's up to the dice, oh and you of course.
This is just an example, it could be anything really, from guarding a king to gathering giant bat guano for a crotchety hermit wizard
Bah I'm rambling again...