The weekend before last, I ponied my younger horse out on trail. Well, the owner of my ranch did the actual pony-ing. Myself and one of the ladies there I'm friends with came along on his stable horses-- he has a whole rental herd in addition to the boarder's horses.
I was terrified before we went because I haven't been on a horse except my own in a couple years now, and I haven't even been on my own in at least two months. But-- I did fine. The rental horse didn't want to western rein at first, so I had to two-hand her, but once we got out on trail she was wonderful. I felt kinda stupid in front of the owner of my ranch and my friend for not being able to get her to go where I wanted for a minute, having ridden for 12 years, but she figured out pretty quick I wasn't the average rental rider and was absolutely perfect for the rest of the ride.
My point is, if you're going out on horses who are used to it, you'll be fine. They've done this a billion times, and they are even less worried about you than you are. Also, if the foundation is there, it will come back to you once you're on the horse.
Have fun-- it sounds like it will be!