Tigerfeet - that's good going. People often talk about 4 miles per hour being average walking pace, but it's actually very quick. I did the Moonwalk a couple of years ago and it is hard to keep a fast pace for a long time without lots of training. The aches and pains will go as your body gets more used to it.

One thing I (and friends who were training) used to experience is that our hands used to swell up in the beginning. Disconcerting and uncomfortable but it gets better the more you train. It might not happen to you but just thought I would mention it - if it happens don't worry about it. Good luck!