Fitness takes time people!
We talk about the desirability to build an "adjustable canter" into a horse, both for dressage and for jumping.
This is a "spring-loaded" type of canter, with hocks under, creating lift rather than, or at least in addition to, push. Most horses tend to find this "something of a struggle."
When I was on the Dartmouth College wrestling team, about 55 years ago, a fun thing our coach had us do was to run up and down the steps of the Dartmouth football stadium for strength and fitness.
I found this "something of a struggle," as in appallingly hard, especially about the third time up those damn steps.
So don't be surprised if you have a horse that would rather coast along on his forehand, rather than carry himself. Engagement , the act of stepping under and lifting, is hard, hard, hard.
Don't think so? Go to your local football stadium, and have a go, for some added insight into what the horse is feeling.