Do something in addition to posting; like do a figure of some kind or work at making a good circle. In other words, engage your mind and your horse's mind instead of just going around the rail.
Posting the trot can really free up the hrose to move out more. IF you sit the trot and don't know how to do it really well (I don't), this will tend to dampen down the horse's trot, posting it will free it to be more energetic and ground covering. Once you learn how to post the trot easily, you will be able to do it in your sleep and go for miles without being tired.
I am just the opposite, I can sit the trot for short periods of time but don't find it comfortable to want to do it for long.