The most important thing to consider is that your dog needs to want to stay with you on the trail, but not too much. A good riding dog will stay on your tail but at their own pace. Some dogs will run themselves into the ground trying to keep up with you no matter how fast you go. You need to make sure your dog isn't running beyond its limits and that may involve going much slower than you might want to ride.
You also need to consider other trail users. Your dog needs to know how to stay out of the way of other trail users, and your dog needs to be well behaved enough to leave them alone. You should always clean up after your dog and you should also consider the local leash laws where you are riding.
I have had some great rides with dogs, and some not so great. There's nothing wrong with giving it a try, you just need go into it with the realization that your dog may not be the perfect riding companion.