1. How good is a mules endurance? I weigh about 90 pounds. So a 3 hour trip + a western saddle and 90 pound rider, = disaster? A mule's endurance is a lot better than a horse's. A mule can go for days after a horse already died from exhaustion (an exaggeration...but not really). Mules are capable of carrying much more weight than a horse of comparable size as well. I weigh about 130, plus my 43 pound western saddle and total weight when I ride is ~175. I am currently riding a mule that is likely the same general size that you'll be riding (about 13.2 and maybe 700 pounds). He can carry me all day long (literally 8+ hours) and still have enough spitfire to try to poot me off when something scares him. Oh, and don't let that scare you, I'm breaking this mule to ride and he's only been ridden about 20 times. That's why he acts up and spooks. Those mules that they use in the canyon will be much better broke and experienced.
2. Should I wear a helmet I noticed that NO ONE was wearing a helmet. ( But I guess that is because they are not horse people
) Only if you want to. If you would feel more comfortable in a helmet, then by all means, wear one. Don't concern yourself with what everyone else is doing.
3. Has anyone ever done this before? I have no idea of what to expect. I've never rode the Grand Canyon (I'm terrified of heights), but I've ridden some pretty rough country. Just stick with your mule, he'll take care of himself and you'll be okay so long as you stay with him.
4. I have been riding for about 6 months now, so I should have enough experience, right? You betcha. Most the folks they get down there have never even seen an equine before, let alone ridden one.
5. What should I bring on the ride? Water is never a bad idea, especially considering the heat and the time of year. I would invest in some sort of light colored hat to keep your head from getting too hot (and it would lower your risk of sunburning your face). I would wear long pants and proper footwear. If you would want to, you could probably take a snack bar as well, just in case.
6. Do you think the mule would have any trouble with footing? Mules are notoriously more sure footed than horses. They can safely walk on a narrower ledge and never put a foot wrong. At times it may feel like they are walking especially close to the edge if the wall is close, but I can guarantee you that the mule won't fall. Where mules get along fine and horses have problems is that, on a narrow ledge, horses will crowd the wall trying to get away from the drop-off. When they do that, they overbalance and lose track of their footing. A mule keeps his body where it needs to be to ensure good footing.
Just go along and have fun. Oh, and take pictures and share with those of us who are too cowardly to ever do something like that. I'd try it and my mule would be fine, but I'd faint and fall down the wall of the canyon or something