A common mistake made by beginning riders is that the range of the trot is slow to fast, when the scale actually goes small to large.This is why most first and some second level tests we see usually have horses running rather than lengthening. It is not about the speed of the gait but the size of it.
low, wide oxers are a good exercise to teach your horse to stretch over the jumps. Having only one stride between each is a good exercise to teach rider to keep their horse collected so that they can get their horse to the right distance in order to be able clear the width of the oxer successfully. #schoolingsecrets