That's great, thank you for taking the time. The course just says to submit whatever you think represents your composing skills, so I might even cut and edit this up into two or three shorter bits and submit them as separate pieces, no-one would ever know.
There are a few key ideas running through the whole thing but maybe they're not dense enough and/or good enough to sustain interest for that length of time. It's bookended by two similar sections totally different in tone which is why I started with the simple first section but I realised it was a risk.
If I was asked to identify my favourite three or four parts I could do that fairly easily, and make sure they were included in the submission. Some of the better bits are towards the end but I agree that if interest is lost early on then they'll never get heard, I've done it when listening to others' pieces myself, and with books and films for that matter!