Remix shortens my music rather than making it longer

It should work fine, but with that sort of extension, you should expect some (okay, a lot) of repetition of any short phrases it finds - that would be typical behaviour. If I try and extend a 30" piece to 3'00" it looks like this:

If the analysis doesn't reveal anything that it thinks it can loop, I'm not sure what will happen, though. I suspect that the results at any given time are very content-dependent.