Sorry Stakie-I agree that this is an accident waiting to happen. I have done several trail trials, group rides, etc, and every single one, no matter how well organized has had one or two who go "whizzing" by, which I also find unnerving. ( and rude) Put that all together, with unfamiliar horses, inexperienced rider, and nervousness, which you clearly are, (and will be if you hear hooves coming up behind you), and nothing good is going to happen here. keep in mind there may also be areas that are too narrow to allow a lot of room to pass. Shoot-I have a horse I could put a monkey on on a walking trail and it still makes me nervous when folks pass.
Best advice I can give-donate the $$ to the cause, take the horses and BF to a beautiful park, and the 2 of you have a lovely ride. I am guessing you would like your fiance to have a good experience so that he may be likely to do this again in the future......the organized ride may not be the way to start.
I would highly recommend a cell phone-ATTACHED TO ONE OF YOU, not the horse, also.
I would also be asking the actual owners of the horses you are going to use, rather than a bunch of strangers who do not know you, them or what the trails may be like.
Have fun-whatever you do. That should be the main reason for riding, IMO. You can always donate to a cause without making yourself a basket case.
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