As a lot of people told yet; I don't think it would be a problem.
Or else I'm doing something very very wrong with my horse...
When training our horses we often say; they feel good, and their basic condition is fine enough when the horses are trotting easily for an our during a ride outstide.
In training most of the times we interrupt the trotting bij a galop or walk. So more then an our is not a daily thing. During an competition (endurance), it all depends on the horse. Sjors (welsh pony) likes to galop, just like Kamin (arabian) does. But Yuri (arabian) can't perform long in a galop. He trots the whole distance.
And he does it well! Now he is about 14 years I think. He has been walking 160kilometers two times now. And of course all the other competitions (90 kilometers, 125 kilometers, 60 kilometers) before this king distance. And not to forget the training kilometers. All the kilometers he does in competition are in trot. (and a few in walk of course).
Of course you can't just go on trotting and expect everything to be fine. A horse needs to trot with his weight balanced over his 4 legs (so he has to carry enough weight with the backhinds to ). And also the horse has to be supple! So practising the muscles is even as nessecairy as making training distances.