1.) You shouldn't factor an average speed change from just one ride. Especially when your previous number consisted of data gleaned from several years of riding. So, not really.

2.) It shouldn't, it's extremely difficult to use a fixed gear to follow a structured training plan but if you're just riding a lot and trying to push yourself without being technical you should be fine.

3.) Yes, it was fun, my butt hurt. This link has some good information if you're curious: http://www.bikeforums.net/showthread...d-Gear-Century