I used to saddle up at the start of the day for a show or pony club and leave my horse saddled all day (loosening the girth if I wasn't riding) until after the last ridden lesson (or class) was over. Saved time, didn't bother the horse, I would only unsaddle to switch saddles if needed. But that's because I'm mainly interested in eventing and showjumping, so I need two different saddles, a dressage and a jumping saddle. So if I had a flat lesson, then a break, then a jumping lesson, I'd swap saddles during the break. If I didn't have a break I'd saddle up with the jumping saddle and just flat in that.
At particularly busy shows I'll eat on horseback (carefully, so as not to get food on my show clothes).
My horses are usually very fit by the time it comes time for showing though so it doesn't bother them to have a rider on for 6 to 8 hours straight.
Edit; the above is only relevant to the 'leaving saddles on' question, and has absolutely zip to do with trail riding. However, one of my horses would willingly go all day on trail rides, and usually still had enough in the tank when we got home for more work.
MAKORA THOROUGHBRED SPORTHORSES