I was lucky enough to grow up with ponies, but many kids will only have the opportunity to try horseback riding on vacation.
My family did the trail ride in June 2016. It was not linked to My Disney Experience even though we made the reservation well in advance. We also did it back in 2007. As others have said, it is completely suitable for beginners with a guide in front and one at the back. There is also a 3rd guide on horseback who can move up and down the line. I have been on other trail rides and this was the most organized one. I was directly behind the front guide both times and enjoyed talking with them about horses. My family really enjoys the leisurely ride through Fort Wilderness. It provides a nice break from the parks.
We also visited the pony corral. All of the pony ride ponies are pretty small.Disney should get a 2 or 3 larger ponies for the taller young kids. I would have been excluded at 8, too (and maybe even 7, I was really tall for my age).