For a long ride, you're going to need someplace to hang certain things. I never go out on trail without a halter under the bridle and a long lead attached, which I coil around the saddle horn. I like to have a place to tie on a rain slicker and a jacket or long-sleeved shirt. You will need to hang a canteen somewhere, either on the saddle itself, or in a saddle bag. I suppose a camelback would do the trick as well, but on a long ride, that would become uncomfortable, hot, and create a sweaty spot on me. Where will you carry your hoof pick (you do take one with you, right?)?
This is the saddle I use on Dream (and the one I commented about riding thousands of miles in):
And this is my catch-ride saddle, that goes on anything else I may be lucky enough to ride:
Riding English pretty much requires constant contact with the horse's mouth. I would think that would get tiresome for horse and rider after four or five hours (although not being an English rider, I may be wrong on that). Western riding is generally done with one hand on the reins, allowing a free hand for other duties, such as towing a pack horse or handling a camera or binoculars.