Well, I feel that firstly you need to know that any dog trained for personal protection is not a pet. They are a liability. If they bite someone you did not intend them to, you will be in very serious trouble, and this can even happen if the dog attacked someone you meant for it too. Personal protection dogs have even been known to attack their handlers if they feel they're being threatened because they are taught to protect themselves as well.
Any dog trained for legit protection needs years of professional training and experienced handling.
But if you're looking for a watch dog and something that may turn strange people off from you, they're likely to get the job done. They can be a pretty intimidating dog.
As for endurance, yes they definitely have that. They definitely have the energy level and physical ability to go along on trail ride. Being a hound, they may have a tenancy to wander though, so teaching them recall would very key. But I would say if you socialize one with horses properly they'd do fine.