...and once your butt stops aching, and you've doubled your distance, and your calves will be a mess, and that'll heal, and your hands'll ache, and you'll be at 40 milers, and you'll start obsessing over KOPS, then you'll buy clothes you never expected to, and the inside of your left knee will feel a twinge, and you'll spend 3 days adjusting fore/aft seat position, then it's clipless shoes and pedals, and your at 75 miles, and starting to think of updating your bike, and....
IOW
Your doing fine, just keep doing it.