Contemplating length

Have you ever written a short story?

The more I read about publication options in SF/F, the more I keep hearing about writers who started with short stories. Seems to be the "thing to do". Write (and hopefully publish) a few shorts in the same world as your novel. Build the fanbase before you even sell the book.

Except...

What if I can't write anything that short?

I didn't start out writing short. Yes, I've written a few in my time. For various classes, assignments outlined by one teacher or another. But when I started out to Write (with a capital W) a few years ago, I tackled a novel. My first real, self-directed work of fiction was a NaNoWriMo 50,000 word contemporary romance that after edits and rewrites has ranged from 65,000 to 80,000 words, depending on the version. Since then I've "finished" two additional manuscripts, one at about 55,000 words and one at 90,000.

5-15,000 words? What on earth (or rather, what in space) can I accomplish in that short of page count? And is it really worth my time?

Maybe, just maybe, my brain will work better in a longer format. Maybe I will just be a novel writer. Or am I shortchanging myself?

Intiuitively, I believe that writing (and any other brain activity) is a lot like physical exercise. You have to add variety. You have to change things up. If you don't, you fall into a rut and start losing ground. Your body gets too efficient at one thing (running or bicep curls or biking or whatever), and stops having to work at it, and you plateau in your performance or weight loss or development. But if you change up your workout, and keep your muscles guessing about their next challenge, then your favorite/best/core activites get even better.
So, should I try to write short? Should I try to come up with story ideas that can be fleshed out in less than 50,000 words? I might have to *gasp* lose the romance angle if I do that. Can I write a story without a happily ever after? Or can I build up to one in such a short work?

Part of me thinks yes. Yes I can. Yes I should.

But then part of me screams No. If I divert my attention away from the "real" writing, will I ever finish manuscript number 4 (or 4 and 5, seeing as how I have another partial one in limbo on my harddrive from 2 years ago)? And what happens if I never sell the short stories either? (that's me, ever the optimist about my publication future)