Hello and welcome! I am a newbie too. I have done one 25 and will be riding my second this Saturday.
Don’t make any major changes to tack until you know you will like/love it enough to keep doing it.
The best way to train is to ride! You don’t train for a 25 or 50 by riding that. Usually I ride 10 or 12 miles. I only get to ride 2 maybe 3 times a week. If it’s just me, I will try to gait (I ride a fox trotter) and lope most of that. If someone else is with me, we probably walk half of it. My husband’s QH could not keep up that kind of pace. Get to know your horse so you know if you are pushing him too much during a ride.
Shod versus boots really depends on the rider. I like my horse barefoot so will get boots if there are any rides that require footwear. But I am in Texas. Most of the endurance in my area of the state are barefoot friendly.
I hear you can’t have enough buckets at camp! I took bare minimum supplies and slept in my truck. I just tied my horse to the trailer. If I keep doing this, I will probably invest in an electric pen. I brought a cooler with lots of water and brought sandwiches. I wanted something quick and easy. I didn’t have anyone with me so was glad I did bring sandwiches when I was on my hold. By the time I had taken care of my horse, there really wasn’t much time to eat so having something quick and ready but great.
You don’t need heart rate monitors or really any of the “fancy” equipment a lot have. They are nice but not needed. Especially don’t buy them before knowing you are going to stay in the sport.
I rode in sweat pants. I"m thinking of wearing them again. I’m going to be in the saddle for several hours and want to be comfortable. I do have a treeless saddle. I didn’t get it for endurance so much as because my horse is hard to fit. I used a nylon bridle with braided reins. I have recently gotten an Indian hackamore. That’s more because I want to get away from using a bit on this horse. He is white. Even when not pulling on the reins, he gets rubs at the corners of his mouth. It is something I would have bought whether I was riding endurance or just trail riding.
Why wait 2 years? If you are regularly riding, Trigger should be able to handle an LD.