I think a lot of people fall into the same comfortable heartrate and speed ruts as they ride. If you want to improve, you need to push yourself out of these comfort zones. Even if you had some intervals into your long weekend rides, that should start to push you in the right direction. Otherwise, I think including a ride during the week will help, even if it's just short and fast or a challenging group ride. And there's nothing wrong with 12 mph! Most people just aim to finish their first century. As they do more, they might set a time goal, but don't put too much pressure on yourself to do that. Just have fun!