15mph pace on mixed roads on a mountain bike is a good pace. Try for an hour a day 5 days a week to start. Then add a longer ride 2 or 3 hours each weekend. Pretty soon you will be knocking out 100k in a day just for fun. It is about saddle time, not speed at first.

Once you have some base miles you will need to change it up a bit on your daily rides. You will start looking for hilly routes, easy recovery loops, and places where you can do interval training. Then speed will start to happen.