I have a 10 yo mare that I've been working with
since November. She had been a brood mare and really wasn't ridden too
much previous to me buying her. I have been working with a trainer and
doing some trail riding, but really wasn't thinking of taking her out on the
e-trail until next year. But....., my main endurance mare is having some
tendon issues so is on rest. I was going to do a 50 with my main mare
this coming weekend. About two weeks ago the plan changed because of the
need to rest to going ahead and taking the 10 yo to the ride to do a 25
instead. She is a little older than the horse you are talking about, but
still pretty new to all this endurance stuff. I plan on riding with a
partner, taking it slow and am not even concerned if we finish over time or
not. I just want her to have a good experience and get her broke into the
sport. So, sure, go for it. Have fun, take it slow and don't fuss
over the time, just make it a good ride for you and your horse.