Some of the pain and soreness (especially your butt) will go away. You're using muscle groups that you probably aren't used to using, and 7 miles is good for a first ride. You'll eventually work your way up in mileage. It took me over a year before I did a ride over 30 miles.
Also, don't worry too much about average speed. It can vary a lot, so don't worry if you notice that it's low one day. However, I would try focusing on your cadence. 72 is pretty slow, but a lot of new cyclists pedal too slowly. It's something that feels unnatural, until you get the hang of it. 90 rpm cadence feels natural to me know.