Before you leave, remove as many practical unknowns as possible.
Know how you are travelling, what the weather looks like, what footwear the terrain needs and what the event page says about pace or difficulty. Practical uncertainty can easily get mixed up with social nerves.
Pack what you need the night before if that helps. The less admin you are doing on the way there, the more headspace you have for the actual day.
When you arrive, do not judge the whole event from the first sixty seconds.
People are finding each other, checking names, sorting bags and waiting for late arrivals. That can feel awkward because nothing has started yet.
Give the event enough time to become the event. Once people start walking, the group usually becomes much easier to move around and talk within.
Your first goal can be tiny.
Learn two names. Have one proper conversation. Stay until the first natural break. Those are all successful first-meet goals.
You are not auditioning for a friendship group. You are giving yourself a first point of familiarity for the next time.