I would look at appaloosas, QH, paints, or any of the above crossed with arabs. I am not as familiar with other breeds as I am with apps, but I know that most of the ones I've had can go all day.
We had a stud horse that worked 8-12 hours a day on the farm and had no soundness issues. (I'm sure this is true of many other breeds, too.) One of my favorite things about him was that I could pull him and a couple of the mares out of the pasture, throw on some saddles, and go for a relaxing trail ride. Some of that was training, but most of it was disposition.
There are horses in every breed that are harder to handle than others. I wish you luck in finding the right horse for you!
Learning never stops