Comics are more than just drawing well, you need to be able to tell a story clearly. If you can do that with drawings alone (and little reliance of dialog), then you're on the right path. A lot of film theory applies to comics... continuity, blocking, camera, location tagging, the 180-degree rule.
And you're right - there is a vast amount of styles (and stories) in comics! I have a hard time being consistent myself... but it is crucial for a comic to have a fairly stylistic consistency.
You say you want to go for a realistic look, that's pretty broad too.... from the more painterly artists as Alex Ross and Jon J Muth, or supertight pencils as Jim Lee or Bernie Wrightson.
In my opinion (and my teacher's when mentoring me about style), it's best to focus less on style - draw what comes naturally to you in what is "realism" to you (or what comes naturally to you as "stylized" if you go that path), and in the comics world, focus on clear storytelling (which also means good compositions and clear drawings, good continuity and blocking). Study your favorite artists, see how they handle certain things, but do it with your own natural instincts and sensibilities.
Speeding up your drawing time:
- First off, having a good foundation and theoretical understanding in anatomy and perspective is important. Good... not perfect. wink You can learn as you go.
- Always begin with thumbnails. Good thumbnails help you decide the layout of the page, blocking, composition, and camera/perspective.
- When you decide on your thumbnails, in drawing for good layouts, references can help significantly with drawing time, and will also help you learn better ... you may get to a point where you'll rely on them less and less (but every professional will have 'em). Get your friends to pose for you in the position of your rough layouts, so you can get them down correctly. Consider costuming them too to best resemble your characters outfits (for correct clothing folds). Use stairs to help with shots looking down.
You should gather references anyway for your backdrops and world-building. Look at movies and other comics to see their compositional and camera choices.
Anyway, instead of learning through short comic strips, go for actual pages. I learned in school by doing a minimum of 3 pages for a short story... which makes sense in a storytelling sense (intro/beginning, middle, and end). I personally think you'll learn better that way, especially in gathering some of the crucial aspects of sequential art.