an arabian mule? what does that combo look like...haha i bet its strangely cute :)
Arab Mules are super cute! I have seen one.
IMHO, you are ahead of the game with a dressage base. It is good to have excellent control and be able to rate pace and stride out on the trails. I don't think there is really any age that is too old to start, but definitely take it slow, and, if say your horse was 19 and totally unconditioned, I would be saying you might want to reconsider unless the vet gave you a green light.
If your mare hasn't done a lot of hacks, you could have some spooky issues so really start with long, working walks just to make sure she is OK with some of the 'scary' stuff out there on trails, you know, like bushes and rocks (lol). Endurance is not only physical, it is mental, too, for the rider and the horse. The horse needs to build up the mental stamina to pay attention for hours and hours and not get bored, frustrated (getting passed by horses can build horsey angst if they aren't used to it) and then stops eating/drinking. I worked with a good friend and we would have our horses "leap frog" each other so they wouldn't get panicked or change pace when passed. Very helpful.
I think the best thing I did to start was long working walks, and I carried baby carrots in my pocket (still do). Whenever my mare drinks water at a water crossing or station, I gave her a baby carrot - she's pretty smart so she got the picture and hydration isn't too much of an issue but that can be a problem if your horse isn't normally a good drinker (when you give the carrot from the saddle, alternate sides and it will help them stretch out their necks, too). It helps to ask them to eat at places so they get used to when you stop, it is time to refuel. Many horses have trouble with these on the trail because they get worked up and that leads to metabolic issues. But after good working walks build up to trotting and trotting for long periods of time (every horse is totally different so there is not a set "start trotting now" point - tracking resting heart rate with a stethoscope will help you figure out her condition). When you can mostly trot a good "loop" (10-15 miles) and have a the heart rate drop below 60 relatively soon after stopping, you are good to go.