Average speed vs. average moving speed = total distace covered/total time taken vs. total distance covered/moving time (subtracting the time spent stopped, e.g. traffic lights).

Sore butt and other body parts the first time on a bike after a long pause is perfectly normal, it should subside within few rides. Padded shorts is individual choice, they may be useful for long rides, although I'm doing rides over 100km in regular clothing without any troubles. It'st also possible that the saddle is not the best fit for you, but give it some time; you should consider changing it only if it is dialed in as good as possible yet the soreness still doesn't get less even after a couple weeks of more or less regular riding. Your fitness will also most probably increase noticeably within first few months of riding, if you stick to it.