It would be a wise idea to also get horse endurance training in addition to just riding. Riding a horse, even for a professional, can be taxing on the rider-- especially for long periods of time. It's not just a matter of sitting on the horse and letting it do all the work; there are lots of balancing and use of your muscles, too, not to mention having your legs spread open all day (just being honest there). Having little experience, you'll more than likely wake up sore and achy and riding will become more of a pain, and less of a fun and historical experience.
But it does sound like an amazingly fun trip, and I wish you luck!
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