I find (and Heather teaches) that to lengthen the leg, we need to open up/relax the hips. One of the best ways to do that on horseback is to take your feet out of your stirrups (assuming you are safe to do so) and ride along with the toes pointing actively downwards, like a ballet dancer en pointe. I'd start that off in walk, and then just two or three strides in trot at a time. This shouldn't put too much stress on your horse, and should help strengthen his/her back too.
What saddle you're using can also impact this...knee rolls etc can be very helpful for keeping your legs in a more suitable position