I am looking at a couple of rides here in Texas and trying to learn as much about the sport as possible.
I last rode in an endurance ride in the mid 90's on a 16 year old gelding that Had never been on one and enjoyed it. It was only 25 miles but had a lot of fun.
Now that I am semi retired and have all these wonderful Arabians I am thinking about trying it again.
Best way to know for sure about if helmets are required at a ride is to contact the ride manager (their contact info should be listed on the AERC regional ride calendar for your area). Should be a simple yes or no answer.
At least in this region, the distance you are riding (LD vs longer) doesn't make a difference in terms of helmet requirements. Even people riding "just the LD" (and I say that very tongue-in-cheek as LD vs "real" endurance is a whole other silly debate) need helmets in this region.