Depends...what does "at a time" mean? How often do you ride? Is 12-15 miles your normal distance?
What are your stats? Age, Weight, General Physical Condition, Medical Issues, etc. They all matter to a degree.
What do you mean when you say you are "struggling at the end"?
How long does it take you to ride that average distance?
Rather than try to ride at the fastest average speed do harder and easier efforts to train your mind and body for the harder and recover during the easier.
Runners use a training effort called "Fartlekking"...spelling??? Basically it is non-regimented run of easier and harder efforts...run hard when you want and then easy to recover. It can make 'training' a bit more fun than using a regimented method especially for newer riders, etc.
You can use a hill for a hard effort...you know it is coming so ride at an easy pace then go into the hill a bit hard and continue to increase the effort until it is too difficult then sit and spin...recover then do something else like sprint for a town line sign, etc. You're training your body and mind to become stronger and get used to harder efforts knowing an easier recovery period is on the way.
Incorporating the above you should find that the ride you mention goes faster and feels easier after several weeks...it is not an overnight thing.
When the ride become easier you can add another couple of miles and perhaps more 'hard' efforts...mix the hard efforts with medium hard, more hard for less time...perhaps 30 seconds. Also mix how you tackle the hard efforts, meaning do some from a seated position, some from a standing such as a 15 second sprint or a short hill and some from mixed, meaning you start from a standing effort for perhaps 10-15 seconds then sit and hold that effort for another 15 seconds.
Good luck and keep us informed...