You don't really improve your trot lengthenings by working directly on them, per se. You improve them by doing exercises which increase the engagement in your horse's hind legs - half halts, the almost walk exercise, etc. When you have more engagement in your horse's trot, try your lengthening again, and it is likely that it will be improved.
If you are hoping to improve the scores on your medium and extended gaits, improve your collected gaits.
From Facebook fan Dorothy Smith ~ "In riding, as In a dance, one leads, one follows, but both must love to dance for it to become art."
Give your older horses even more time to fully warm up before any challenging or strenuous work.
"Sit on your bum. If you do that you’re part of the movement. If you squeeze with your thighs you can’t sit. Stay soft in your hips, draw yourself into the saddle and keep the calf just against his rib cage." ~ Bettina Hoy