I did this! It was awesome. The mules are extremely well trained. You don't have to do anything much, just sit there and enjoy the ride. We had people with us who had never been on a horse or mule in their life and they did fine. They stop the mules quite often to let them rest.
If you are taking the 3 hour ride you are likely doing the halfway trip, instead of going down to Phantom Ranch. They will give you a riding crop because the only thing is the mules can tend to lollygag. Then the trail boss will have you tap them if they do to make them catch up to the others.
You will be given canteens and every time you stop they will tell you to drink. Do so even if you are not thirsty, the dry heat will dehydrate you fast. Don't be surprised when you get to the halfway point and stop for lunch, they will take a water hose and drench you, the mule, the saddle and everything. You will dry quickly, LOL! But that water is cold.
Have fun and take lots of pictures and videos, the scenery is awesome.
Oh and by the way, the weight limit I think is 190 pounds.