I say stick with the shorter distance for your first ride. It will be less stress for you and the horse. You think to yourself, Ahh... It's just 15 more miles, i'll just do the 50. Well when you are out on trail and tired ( you and the horse) 5 more miles seems like an eternity. Lol. Thankfully I had a mentor and started out on her horses who were pro's at endurance when I came along. I compete in 50 to 100 milers and if I were starting over with what I know now and did not have a mentor, there is no way I would do 50 on my first ride. Just do the 35 and go have fun. So what if you decide you need to pull, so what if you go over time. More importantly is the experience you get from it. By no means am I saying you want to pull or go over, but it happens. Just keep a watch on your time and keep it steady. Think Steady Eddie. And for the most part there will probably be someone there you can buddy up with or get pointers from. I am all about helping the newbies! I want them to have a blast and their horses to have fun also, so they keep coming. Endurance, believe it or not intimidates a lot of people that would like to try it, but don't have anyone to teach them. I don't know about other regions, but my region has a first time riders meeting after the riders meeting the night before the ride. Maybe your region has something like that?
Anyways, listen to your horse, watch your time and most importantly- Have fun!!! Good luck!