Time, poles, hills.
It takes time for a green horse to develop the physical strength to sustain a canter.
Pole work, and there are plenty of online resources available with patterns and exercises, assists in many areas of training: suppleness, balance, teaching the horse where to put its feet, getting it to think more quickly, improving suspension, adjustability etc etc and is also interesting to do for both horse and rider. There are just so many useful things to do with poles.
Hills are ideal for building muscles and stamina and balance, both going up and going down. Getting the horse out of the arena into the big wide world is hugely beneficial for opening up the stride, getting the horse thinking and moving forward and having FUN. Fun is often forgotten or ignored in training horses.