Trotting IMO the one of the best ways to build muscle! When I was eventing, trot work was 90% of our work. Builds great topline, and develops your horse's fitness and stamina.
But it must be quality trot work (not suggesting you would do otherwise
) What I mean by that is it must be active and sustained. I would do 20 mins to half an hour of 'long trot' work out on the trails. I would push my horses to trot at the biggest trot they can sustain before cantering and then maintain that for the full workout session. It is very hard work for them, if you are doing this correctly, they will probably try and canter as it is easier for them, but keep them to a trot.
Good workout session for the human too, it is very energy consuming! Before you get to the full twenty minutes, start with smaller bursts and give a couple of minutes rest. Over a period of a few weeks, build your horses fitness up to a point where they are able to last the distance. If this is something you are just starting to do, you will need to condition them to be able to sustain the work to avoid tendon/ligament/muscle damage.
Whole lotta fun too!