I am actually a published composer, a couple of small things for brass (not "self"-published.) A few more that have been played locally and elsewhere and gotten good reviews. And one that has been played by a local well known concert band (a march.) I am a classical musician, and that is the genre I write in. But here's the rub: I know I have good material, but I haven't a clue how to "lengthen" a piece. I tend to write little ditties that last a minute or a few, and despite a degree in music and having taken a couple of college courses in composition (useless; it was "bring in what you wrote so we can comment on it") there is a piece of the puzzle I'm missing. I don't want to write a symphony but I'd like to be able to take themes I know are good, and incorporate them into a piece longer than three minutes. The only thing I can think of to date is to take a piece I like, and literally copy the composer's structure using my own material, but it feels like plagiarism to do that. I am proud of the things I have in print, but the march in particular was an exercise to see if I could write a march, and it is, in my mind, "very" mediocre and was put in concert band form because the brass quintet version sounded pretty good and the band director asked me to arrange it for band. I want to be able to take things I'm proud of and make them longer.
First take encouragement in the fact that many movements clock in at under three minutes. Look at early classical pieces or baroque string concertos for examples.
Using a great composer's structure as a guide seems legitimate. It was recommended by Czerny through a quote attributed to Haydn.
But I think the basic answer is to gain an understanding of the various forms and the typical duration you get with them.
A sixteen bar minuet with repeats gives you maybe a minute, minute and one half. Extend that simple dance form to 'da capo' or ternary form and you can get something around 2 and a half minutes.
6 variations on a 30 second theme, there you have 3 minutes.
Take a sixteen bar dance form, insert a few extra bars of themes grouped into tonic/dominant areas, write a recapitulation after the double bar repeat and you have a sonatina. That will give you a longer duration.
The point here is various forms suggest different sizes. Maybe study up on the larger forms like sonata or ternary. One book I suggest is "Classical Form" by Caplin. http://a.co/5XScBSI. It takes the reader through various forms starting with small forms and gradually building up to larger forms. Also, "Fundamental of Musical Composition" by Schoenberg may be helpful. http://a.co/4mUWnSc.
Making things longer usually means variations and inversions of a theme, structures, progressions, etc. This is where compositional theory can really help you. Try taking what you have written and then weaving it into an inversion that follows. Or a counterpoint. Or a key change or diatonic shift. These are all devices that composers use to vary their material.
Beethoven's Fifth is really just 4 notes when you boil it down to essence. But listen to how he took those 4 notes and expanded them throughout the instrumentation and variations in the first movement.
Four suggestions: You could always create a suite. You've already done most of the hard work. If your themes are strong, repeat and reharmonise them. Sonata form is a wonderful structure: it is a principle, a lattice and even a set of instructions. You could do worse than giving it a go. Since servants went out of fashion we've become more time-poor.People don't seem to go in for powdered wigs,big shoe buckles, petting spaniels and 'off with his head' if a piece is under 20 minutes long. I wouldn't be in too much of a rush to crank out epics just for the sake of it.
Copying structure from existing works is an EXCELLENT idea!
With brass groups and concert bands you're working for an audience that appreciates a 'good tune' rather than cleverly developed themes. Nothing wrong with that. My definition of Purgatory might include listening to 'art music' performed by a community concert band (and if it was one of the crack military ensembles it wouldn't be a lot better)!
Maybe not a good idea to go further 'out' than the Holst or Vaughan Williams suites. If you're American and don't know these, listen, enjoy and learn!