If your serious about music, get on the internet and start 'googling'. Computer Music is a great magazine for you.
There are a lot of free courses on music on the net.. Study them all, and learn what you can. Start by copying someone else's idea.. Figure out what they did..
You'll find the path that is right for you, but it's going to take work.. I have a friend who is pretty successful, but doesn't know a lot about music.. He uses a lot of samples, and some of his stuff is amazing.. Now many here would scoff at his technique. But there is a place for everybody.
What you propose is not totally unreasonable, if it is the starting point for you. You might be able to find a wanna be mixer or producer, that would fit with you..
Study and analyze music that you like. Find websites that accomodate the Fruity Loops and Ableton Live crowd. Write out an analysis of the song. Drummer plays 15 bars does a roll, then goes to a new beat for 8 bars. etc.. Same guitar riff in 1st and 3rd verses..
Get some graph paper and chart out the songs.. don't worry that you can't read the notes right now, that will come later, if you decide that's how you want to create music..
Count out the measures and draw lines for each instrument, use squiggles, slashes, circles, anything to help you to remember what the instrument did. Do this for every instrument. Checkout the 'idiots guide' books there are available.. You can certainly learn to decipher sections, when and how they repeat.
You have to learn by doing it yourself, or come up with the money to take a course.. Personally most of my life, I've been a do it yourself person.. I did study music for several years when I was young. But the vast majority of my abilities came from working in a recording studio with other musicians.. Different people approach music differently.
Look around there are a size-able number of people in the same boat as you, they are just not here. I write pop music, which I upload to several sites. Some of them the uploaded music is not that good.. Yet, people encourage them. I upload here, but get very few comments. The vast majority of people here are working on a different level and field of music..
My music is not 'legit' enough for some of the members here.. There are some truly brilliant minds here..
But you are going in a different direction, than most here, I suspect. Look around at different sites.. Run an ad in your local Greg's list, you're bound to find someone on the same plane as you.. Good luck.
To make music of certain length, you must have the motif or theme, state it, develop it, restate it, go to a different motif and then come back.. That is one approach. If you only have one 15 second riff, you're gonna have to develop it, mutate it, and something else for variation etc..
In my pop music, I can do about 5 minutes keeping to one general theme, and even then I got some complaints, cause haven't developed it enough.. I don't have the classical training to do it. I listen to other peoples work. Sometimes I feel, I don't even have the 'legitimacy' to comment on it.. But I pick up ideas.. I'm going to a free sight that has classical and jazz courses on it.. I'm focusing on the jazz because that interests me right now..
I also work as a doorman at a club, and the technobabble played, no one here would call music. Yet there is a vast audience for it.. A lot of music created by DJ's have no music training. But they have good ear, and have put in a considerable amount of time listening and playing music.. Noticing what kind of music, what beat, what synth patch, make the audience go nuts.
Like the OLD saying,,, There are many ways to skin a cat - likewise there are many ways to make music..Don't be discouraged.. keep working at it.
Just listened to a couple of your pieces, while not my cup of tea; you have some talent for coming up with 'arresting' ideas.. I'd first start by stringing some of these sections together. Pick one or two of the loops and repeat them.. Put some wild drumbreaks before going to next motif..
Musically yours....