Haven't done Zumba yet - in person or on DVD - but it looks fun. I agree with ARCHIMEDESII -- the more you practice 30 Day Shred (or anything else), the better you'll get. And substituting one cardio move for another, or one ST move for another that hits the same muscle, is something I still do sometimes and I've been exercising regularly for over 6 years now -- it's a good practice for when you're sore or whatever, and it adds variety to help avoid boredom.
FYI, CollageVideo has previews of almost 1000 exercise videos (though I don't think they have Zumba) so you can see if there's something you'd like to add to your collection. Good Luck! www.collagevideo.com/
Edited by: FITVALGAL at: 6/20/2010 (13:13)