MikeJones I do participate in one other outdoor forum but I don't want to advertise in there.
Great, and that's not what you should consider. Nobody likes advertisement, not for commercial purposes and not for competing websites. I thought more about people with whome you have developed a personal bond, people you have already contacted by email or other means beside the forum, people you have probably already met in real life.
If you can find three or four other likeminded souls who see your forum as their own project too, that's worth more than twenty people who have registered due to your advertisement. Registration in your forum is not the endpoint. People engaging with meaningful content, people seeing your forum as one of their homes in the internet, people enjoying to engage with you, that's the endpoint. I have one member in my forum I learnt to know in another forum 15 years ago and we still enjoy our conversations.
I like your forum because it has a real purpose (and an interesting one), a foundation for meaningful content, a basis to have fun together. Now you need to find some people who want to share their experience with you. Not a lot of them, but the right ones. Why not ask someone from your family, some of your real life friends, people you have met on one of your trips?