We were taught lot of long trotting. A lady who excelled in endurance conditioned her arabs with long trotting and hillwork. She rarely cantered her horses as it requires more effort to cover the same distance.
This is what I was taught too, but more current research is actually proving that the reality is the reverse!!
Research is finding that a long
trot, over time/miles, puts a lot of unnatural strain on the shoulders and back of the horse and actually changes the muscle development in a way that is potentially detrimental longterm. I don't have the links to the studies in front of me, but I will look for them when I get home. I know I can feel a difference in how my mare moves between a normal and a long trot, so I can see why it would develop different muscles.
A regular trot makes the best use of the suspension mechanism of a horse's limbs, but a balanced canter (and that is the key, balanced) seems to work best in terms of covering the miles the fastest. Seems the crazy FEI people who canter 100 milers (or the scary Europeans who canter on pavement during rides
) were onto something after all.
I did a 3-horse experiment (with the 3 in my backyard) and found their heart rates were equal or even less at the canter than at the long trot. I was really surprised, but it made me focus on getting my mare to have a nice, balanced canter (trot is def her gait of choice).
To the OP, I think just start picking up the pace on your rides and see how your guy does. I think he's much fitter than you give him credit for!