I was always told that trotting work is far harder than any other gait for a horse...
Doing that on a grade...that much harder for the cardio.
Are you saying you think your horse is not fit because you don't see massive muscle mass?
Does your horse blow hard after you've been riding nonstop for about 10 minutes?
Does the horse not sweat more?
Not every horse makes mass muscle but will have long, lean muscle mass such as you see on a Thoroughbred versus the more bulky of a Quarter Horse.
However, some of those things, such as not blowing hard, sweating excessively are the marks of a horse in good physical condition..
Just like a human who is physically fit it takes a lot more to push them past their fit condition to get them sweating and breathing hard is true for your horse....horses are athletes too..
However, if you are saying your horse is not fit because no matter what you do the animal not sweat that is something very different and needs immediate exam by a vet for Anhidrosis ...the name for not being able to sweat and it can be deadly if left untreated.
Know which it is you are asking for and watching for signs of...