I have troubles composing my first Impromptu, which is in the key of Eb Major. I'm composing it in the form of A-B-A. With the A section being "Andante cantabile". I want the B section to be "Allegro Con Fuoco" with a 16th note run. But i have troubles with composing a 16th note run, is there a specific way to it? Any answers would be much appreciated.
There's really no one way to do it, but consider the high notes of the run as a melody (which need not coincide with the beats), with the other notes as arpeggiated notes from the harmony, using non-harmonic tones as needed. You can then work backwards by blocking in a simple melody and breaking it up into filigree work afterwards.