Join Date: Mar 2012
Location: Gainesville, Fl
Length of the trot stride has to do with conformation as well. A very upright shoulder is going to have a choppier, shorter stride.
I am sure more people will chime in once you post your video and we can see what's going on and then offer some advice.
But, in general, what is going to lengthen that trot is to get his head and neck down (not looking like a giraffe), pushing off the haunches instead of pulling along on the forehand, impulsion (you pushing her in the saddle), and eventually collection and rounding. He needs to have all the proper muscles in her back and haunches built up before he is going to move like an english horse for any long periods of trotting.