For the audio I thoroughly recommend Mark's guide to creating a killer voice over:
http://www.videoforums.co.uk/reviews..._articleid/121
Remember too that the nature of Non Linear Editing (NLE) means that cutting a splicing your video is as easy as, well, copy and paste! Whenever I've made my own instructional videos, I've spent hours on each "section" of the video until each "take" is perfect. Stumble a line or fumble the product? Cut and retake!
20 mins is long, even if you're an ardent fan of PMC pendants. Why not break it into sections? This has the added advantage of looking like you have a lot more guides than you really do.
Just some initial thoughts.