personally, i don't find anything mystical about it all, lol. the actual labour of writing isn't terribly enjoyable for me, but i derive immense amounts of entertainment 'dreaming' up stories, just the way the previous poster does. that's the fun part to me: putting it on paper, refining, editing... it has its ups and downs, but were it possible to get it done without any effort, that's the way for me. of course, i'm lazy by nature (good gawd, i mean i abbreviate the word 'at' for cripes sake). there's a creative urge in me, so, for lack of better means, writing was my most logical option (not suggesting there's any merit in what i write, necessarily). honestly, if i felt i was unable to entertain people i'd give writing up and be a dog groomer or something (at least there's big bucks in that).
i think that's the key for most fiction writers, that they are basically entertaining themselves in the process, too. i'm a hack, so my ideas are primarily without too much pretention, that is i don't feel as if i have to preach the gospel according to the ryan while the world bows in awe of my boundless abilities, and are entertainment-based. it sounds as if you're operating along the same wavelength. not aiming to score with the next 'catcher in the rye', are ya?
write what you want to write about is pretty generic advice, but i can't think of any truer. most would probably disagree with me, but i aver that most writing consists of 75% craft and 25% talent on average. by 'craft,' i mean that you should understand that most likely you're not going to break the bank on ideas, and if you've thought of it once, a million others have already *done* it, but you need to know your writing options nonetheless. for example, there are lots of ways to insert backstory into your novel, and you need to know quite a few. i think that's the craft. relying on talent to make up new ways of exposition, which is new to you and you alone, will only carry you so far.
setting up a writing schedule is probably good advice, too. i ignore this 100%, but that doesn't mean i disagree with it, heh heh. at least in principle. this is basically crap advice that applies to the individual writing it and their personality and perogatives. you may not have that luxury of time. yeah, you'll make time, but in the real world even that's not a sure thing. and honestly, if at the end of your day job you wind up sacrificing significant amounts of time with your family, that's not worth your robot vampire story in space, trust me. and, hey, don't ever say, 'nah, i'd rather stay and write than go on a road trip with a busload of drunk cheerleaders.' i see a lot of writers who all they do it write, yet all their stories suck because they never leave the damn house. be an experience whore. i mean, what memory would you rather have on your deathbed, pitching ball with your son or sitting at the computer? but, obviously you still have to write something somewhere sometime. the bane of my life is that this junk doesn't write itself. oh, to be tom clancy and not have to think of stories that literally can't be gotten out of a plot generator programme.
there's no real average, though i'd estimate most books are written between six months and a year. that you've written for magazines you might be on the six month-side of it. at least you're not some guy who's never strung two sentences together before. and you're older, which almost always means to me the stories will be better. and hopefully easier to write. i give a generic time-frame just to remind you that if you see the novel to completion, it's an idea you're going to live with for quite awhile, so try to pick a subject that has lots of appeal to you. figure out a way to choose the best story idea you've got and start the process. i won't say start at the beginning or to write outlines or concoct detailed character profiles on 3x5 cards, that's up to you. ideally, i guess, writing should be like masturbation, an enjoyable habit.
of course, don't take my advice about it. pick a story and start writing it. damnit.