I used to do DDR 4-6 times a week. I've only played 2 or 3 times in the past year, and that was not on my home machine, so I couldn't track my progress. I played a bunch today, and despite being out of practice, set new records on 4 songs.
I don't think it's being in generally better physical shape, because I get winded just as fast or faster than I used to. My best guess is that ballroom dancing has improved my sense of balance, and that makes my accuracy in time better.
Anyway, it's nice to have a video game where being addicted to it is actually good for me!