I don't think I've ever gotten the hang of GW. I walk in to a town and have no idea what's going on. I have a few quests, but no idea how anything else in the game works.
You can play GW as if it was a challenging single player RPG. The story won't be nearly as good as a Bioware classic but the game mechanics are pretty good.
A simple description:
You go into a town buy stuff from the armorer, weapon-smith and skill trainer. Then you add your mercenaries (AI) of choice from the "add player options" to your party and press the start mission button. After a cut-scene that advances the story you play the mission and it throws you to the next town. You might be able to play the next mission directly from there or you may have to "explore" (maps are pretty linear) outside to find the next town.
At the bottom of Storyline and missions
you have links to the 10-20 towns/missions you have to complete to beat each expansions. The links contain walkthroughs too. Don't read those if you want the game to be more challenging/interesting.
All in all you should be able to get at least 20-30 hours of singleplayer action out of the game.