For pure writing training, nothing, I mean nothing, hones skills like writing short stories. Write one. Write fifty. They're damn hard to write. But you can take characters and plot through a complete arc in a matter of hours, rather than the weeks, months, or years it takes to complete a novel. The end of a short story is nearly always in sight. You'll see ideas through and get practice nailing your themes and concepts to the page. You'll learn about the shape of storytelling in a manageable form.
I've tried to think of a generous, kind way to say this, but the biggest issue young novelists face is knowing when to shut the hell up. Writing short stories teaches economy of words. In this climate of e-presses, of Twitter and flash- and micro-fiction, of stylistic showing over telling, of parsing out POV in sound bites, this is a valuable skill to have. We all aren't gonna be George RR Martin--even the masters among us. Most novelists can't hold their readers' interests for that long.
More importantly, does what you have to say deserve 120K words worth of importance? Take a step back and look at it. What you have to say as a writer is more difficult to develop than learning how to say it. I think sometimes writers start backwards. They focus too much on theme rather than craft, and end up leaving theme dead from poor writing. Think of craft as a way to illuminate theme. Writing short stories forces focus and narrowing of your thought processes. It gets you to practice theme in a safe environment, allowing you to build up to those difficult issues, those Hugo-winning premises that sell books and lock down careers. It's also a great place to test drive ideas without putting out a lot of hours in, because, frankly, you can get your ideas to market a hell of a lot faster in the short form than going over the transom with a novel.
But where to send the damn thing when it's done? Tomorrow, grasshopper, we're going to talk about submissions.