I’ve not done a PMBA Enduro yet but I do the same organiser’s Welsh Enduro Series and the loops are about 12 miles (practice morning, race afternoon). I know the very serious racers don’t take anything with them that isn’t attached to the bike but for me I carry a small Camelbak with 1 tube, water, mini tools, pump and gels. Like I would for trail rides really. Bike is tubeless and I top up sealant before race day.
I wear knee pads and thin arm pads just in case.
I use a Bell Super 2R helmet and leave the chin bar attached but that’s my personal preference as it’s not necessary but you never know… red mist and race day. Well… I know me and it’s necessary.
I do take two sets of clothes/kit with me. One set for practice and anything that gets really sweaty I change at lunch because I just get cold when not moving around. I have tended not to change shorts or shoes but if they got seriously wet / cold I would. I’ve also worn an extra layer and winter gloves for practices in the mornings then raced in more summer gear and less layers in the afternoon.
You can get cold in queues waiting to do a stage. Enduro is loads of fun though and racing really gets the blood flowing. Talk to anyone and everyone you can as you go round. They’ll usually be very friendly and you can have a good natter usually. Start with the guys parked next to you and you might find they’ll go round with you but you can get chatting to others on a climb too and follow them around practice.
Good luck. Have fun. Ride within your limits. Don’t go overboard but be confident and pedal pedal pedal!